This list was taken from user PXG from http://gownation.net/forum/showthread.php?3028-Confirmed-Gears-of-War-3-Weapon-and-Character-Skins. All credit goes to him, I just want to share it here.

Flaming Hammerburst - Unlocked if you played the beta between April 18th and April 24th

Flaming Lancer - Unlocked if you played the beta between April 25th and May 1st

Flaming Retro Lancer - No information at the moment...

Flaming SOS - Unlocked if you played the beta between May 2nd and May 8th

Flaming Gnasher - Unlocked if you played the beta between May 9th and May 15th

Gold Lancer - Play Gears of War 3 online between September 20th and September 27th

Gold Hammerburst - Play Gears of War 3 online between September 20th and September 27th

Gold Retro Lancer - Unlocked in beta (complete 90 matches and get 100 kills with the gold Retro Lancer). Can also be unlocked by getting Veteran Gear in Gears 2 (reach level 100 and win a match on all four Snowblind Maps)

Gold Gnasher - Unlocked by getting Veteran Gear in Gears 2 (reach level 100 and win a match on all four Snowblind Maps)

Gold SOS - Beat Gears of War 1 on Insane

(NOTE - Veteran Gear unlocks the gold skin for all five starting weapons. However, the Gnasher is exclusive to VG)

Chrome Lancer - In Gears 1, finish a ranked match with the highest amount of points.

Chrome Hammerburst - Complete 10 waves of Horde, on any difficulty, on any map, in Gears 2

Chrome Retro Lancer - In Gears 1, complete 10 different co-op chapters as Dominic Santiago on any difficulty

Chrome Gnasher - Complete the campaigns of Gears of War 1, 2 and 3, on any difficulty

Chrome SOS – Have at least one achievement from Gears of War PC (doesn't matter which one)

Onyx Lancer – Get 6000 kills with the Lancer in Gears 3

Onyx Hammerburst – Get 6000 kills with the Hammerburst in Gears 3

Onyx Retro Lancer – Get 6000 kills with the Retro Lancer in Gears 3

Onyx Gnasher – Get 6000 kills with the Gnasher in Gears 3

Onyx SOS – Get 6000 kills with the SOS in Gears 3

Gold Omen (all 5 starting weapons) - Reach level 100 in Gears of War 3

Infected Omen (all 5 starting weapons) - Included with the Gears of War 3 Epic Edition, limited edition console or limited edition controller

Liquid Metal (all 5 starting weapons) - Included with the Gears of War 3 Season Pass

Liquid Green (all 5 starting weapons) - Obtained (for free) through a yet to be announced promotion

White/ Ivory (all 5 starting weapons) - Exclusive to Epic employees or those who are close friends with Joe Graf

Of course, there are many more, most (if not all) of which are unlocked by playing the game.

Characters:

Thrashball Cole/ Cole Train - Unlocked in the beta (play 40 matches and then play another 10 as Thrashball Cole)

Aaron Griffin - "Like" the official Gears of War 3 Facebook page

Commando Dom - Pre-order Gears 3 at GameStop

Mechanic Baird - Pre-order Gears 3 at Best Buy

Savage Kantus - Pre-order Gears 3 on Amazon

Savage Grenadier Elite - Pre-order Gears 3 at Walmart

Adam Fenix - Included with the Gears of War 3 Limited, Epic Edition and console

Michael Berrick - (Assumed) to be included with the Gears of War 3 Season Pass

"Oh yeah!"

Amazon always has crappy unlocks but it's hard to pass up that 10-20 dollar credit

I'm stoked for Gears. But who else just doesn't care about all these skins?

I think I have nearly all of this stuff, just missing the stuff in the Epic Edition and pre-orders.

This game is going to be so good...

Michael Barrick looks great. He may end up being my chosen COG skin.

Pretty much will have all of these except for chrome sawed off as I don't have Gears on PC.

Wow, that's a fucked up amount of shit for Beta folks. I hope it's not Fallout style popups when you start the game, you'll have to wait like 20 minutes for that shit to stop notifying you of your new silly gun skin.

@MordeaniisChaos said:

Wow, that's a fucked up amount of shit for Beta folks. I hope it's not Fallout style popups when you start the game, you'll have to wait like 20 minutes for that shit to stop notifying you of your new silly gun skin.

It won't be.

Fucking stupid pre-order bonuses!

White guns would be cool, too bad it's Epic employee only. 
 
Still happy I got Thrashball Cole in the beta.

Who wants toast!?

@PeasantAbuse said:

White guns would be cool, too bad it's Epic employee only. Still happy I got Thrashball Cole in the beta.

And I don't even think every Epic employee has them, just a few lol.

Long as I can pick Sam, Anya or Baird I'm satisfied.
I don't even remember what period I played the beta in, but I did unlock some stuff I remember popping up.
 
Oh well.

I really like how Gears rewarded you for achievements in other games!

one savage kantus for me then 

Wow, next time do a bit more research when you say it's "official" case n point, http://www.youtube.com/watch?v=VebzUn4A6M8

@Forceman4077 said:

Wow, next time do a bit more research when you say it's "official" case n point, http://www.youtube.com/watch?v=VebzUn4A6M8

Lol, I never said anything was official if read the post.

Your video doesn't work.

That is a disgusting amount of skins. Pre-ordering bonuses are bullshit.

@SamFo: I did as a matter of fact. I don't care if they're weapon skins or character skins, I just see it all as ancillary bullshit. Don't get me wrong, rewarding returning players (from previous games and the beta) seems sensible, but when you need an entire page to keep track of things something has gone horribly wrong.

And yes, Rocksteady's situation is worse, but that doesn't mean this isn't ridiculous.

@applet0n: But if you see it is "ancillary bullshit" why do you care? It's not necessary to have. Just rewarding players for doing things in other games. It may encoruage them to go back to other games.

If it was more like you unlock ACT VI if you've completed gears 1 & 2 on insane, that would be shady.

But meh, it just a little bonus, lighten up :-)

@SamFo: I guess. I understand that it isn't the worst thing in the world, but it just seems pointlessly complicated IMO.

the skins do nothing for me since i don't play multiplayer in Gears, plus i plan on getting it when Best Buy reduces it to 40 for a weekend

I always remember the gold weapons that came with the collector's edition of Gears 2 looking incredibly dumb and ugly. At least these don't sound as bad. 
 
Also, noticed there's no classic Carmines to unlock :(

Just switched from the limited to epic edition so it looks like I'm getting just about everything on that list, fuck ya.

@sofacitysweetheart said:

I always remember the gold weapons that came with the collector's edition of Gears 2 looking incredibly dumb and ugly. At least these don't sound as bad. Also, noticed there's no classic Carmines to unlock :(

The gold weapons look better in Gears 3 and as for the Carmines, I saw them in the Quicklook. I'm not sure if they're unlocked in game or if you have previous Gears achievements.