Ed-e has disappeared, has anyone else had this problem, or a problem with companions going away? It's been in game days and Ed-e still hasn't come back. =(

What platform do you have?  If you have PC I can tell you how to fix this using the console.

Yeah i'm on the PC.

She disappeared in my game too! But I'm playing on 360 =( 
I think it's because of this bug:  
In Vault 22, when you set gas afire while companion is not inside secure computer room, it will be killed permanently (removed from game). You will have to play around with console to get him back.   
I still have her Enhanced Sensors perk, so I guess she is still technically in my party which is a bummer, I can't take someone else because that slot is taken, right? I have a charisma of 7, so I could have more than 1 NPC with me, right? I had Veronica and ED-E, could I have Veronica and Boone if not for the bug?

If you have the PC version you can get a mod for unlimited companions. I have about 6 right now. I like to pretend I'm running my own merc company.

That's a bummer, but there's always robodog. Who I think is better anyhow. Get Lily and robodog and you're basically never getting targeted yourself cus those two are just rushing in (unless you're dealing with 5 or more people).

This happened with me and V, I had to retrace my steps to find her and after I found her I told her to go to the Lucky 38.

same thing happened to me with ed-e after i did his quest and got his gun upgraded it said we was back in primm but i was by goodsprings so i go there tell him to fallow after about 5 min playing i noticed he wasn't there but i still have his perks and i went back to were i found him he wasn't there i looked at primm not there i tried doing the vault 22 trick and the in game waiting trick and nothing happened i,m playing on 360

I lost ED-E as well after getting the upgrades from the Brotherhood of Steel. I manually told him to wait before the entrance of Quarry Junction because the Deathclaws would munch him within seconds while i fight them. 
 
So i knew where he was in the first place. After i finished the slaughter with one hell of a lot Missiles, i played for a dozen of hours without him. Now i want to have Rex as companion, but as you might have read above, that doesn't work if you're with ED-E.
 
I opened the console and entered
player.moveto 001732cf 
(You might want to follow the link of rjb89 posted earlier in this thread if you haven't upgraded ED-E by now)
 
This worked for getting me exactly where i lost ED-E as i remembered BUT he is not there anymore! So now the problem is making him reappear, regardless where he is. I've been to Vault 34 before and Boone/ED-E appeared suddenly. My problem occured after that and the Vault 34-trick seems to work only once, since it does not work anymore.

Any suggestions or solutions?

For me, the quickest non-console way to make lost companions reappear was to go to Vault 34 and use the broken elevator (after you repaired it of course). Once you reach a new floor your companions should reappear infront of you. Make sure that once that happens you talk to them and make them follow you.

Mine dissappeared too, it had some pretty expensive stuff with it.

Guys GOOD NEWS! I went to Vault 22 on the xbox 360, rode the elevator and he reappeared! :D Thanks for the tip Stete.

I play on the 360, and I found that in some situations ED-E can die, but you will still have it as a companion and you will keep the enhanced sensors perk.
 
It happened to me near the Nellis air force base when the boomers shot at me with artillery.  When taking a direct hit, ED-E will simply explode and disappear from the game. It is easy to miss since the dialog does not appear and you keep the perk.
 
I didn't notice until much later. I tried the tricks I found here and elsewhere, including going up and down in elevators, waiting for a very long time, etc. I had to reload an old save to figure out what happened.
 
If you have the same problem and can't make ED-E reappear, you might want to consider reloading an old save since I've read (not sure if this is true) that you will not be able to complete some of the late game questlines otherwise. Those quests require you to leave all companions behind, which you will not be able do.