I played through the entire single player campaign on realistic and while I got the achievements for beating the individual scenes, I didn't get the Realistic Difficulty achievement.  Is it a glitch, or what?

Look through the 'Scene Select' on the Main Menu and make sure all the areas were cleared on Realistic. I'm pretty sure the game bumped my difficulty down at one point without me noticing, but whether it was a weird restart glitch or because I was failing an area multiple times, I don't know.

Same thing happened to me, I've played through twice on realistic and still no achievement...

That sounds pretty fishy. I never heard of a game that bumps your difficulty down without telling you. 
 
I'm having a similar problem; I cleared an entire map on Hunter mode without ever getting detected (realistic mode) and the achievement never unlocked. Luckily there are another five maps, so I might as well try again while beating those.  

" Look through the 'Scene Select' on the Main Menu and make sure all the areas were cleared on Realistic. I'm pretty sure the game bumped my difficulty down at one point without me noticing, but whether it was a weird restart glitch or because I was failing an area multiple times, I don't know. "

Just got the achievement, and the only thing I did different was play the game start to finish in one sitting on realistic.

" Just got the achievement, and the only thing I did different was play the game start to finish in one sitting on realistic. "

Beat most of the game on realistic before I noticed the game auto dropped the difficulty on a couple of missions. Just wrapped those up on realistic and still no achievement =/

That is some major BS that the game turns the difficulty down without informing you.

There were issues like this with Rainbow Six Vegas 2 as well.  Sucks that people are getting this glitched out on them. 

I'm playing it on realistic right now and I sure as shit hope I don't have this issue.

It's a completely common issue, a known bug. I read on the Ubi forums that anyone playing with the game connected an XBOX connected to the Internet is having the same problem. Apparently the only solution right now is to unplug your ethernet cable or wi-fi and play the entire game again on Realistic. Stupid. Still, mods over there say that Ubisoft knows about it and is working on it. I just hope it's retroactive.

I got it without any issues, played through most of the game offline. 
Worst comes to worst at least this Ubisoft is working on it.  That 970 I got from Dark Void still mocks me.

I got screwed out of it also. I hope that the achievement is retroactive when they fix it...although in reality I'll probably end up playing through the campaign at least once more although it could be a while before I get around to it since there's so much other content in the game.

The forums on xbox360achievements.org has threads about this achievement being possibly glitched.

I had the exactly the same problem I defiantly played through the whole campaign on realistic and it didn't give me the achievement. I went on the Ubisoft support forums and people are thinking its related to the update they put out. Some people are clearing their cache and playing through the whole campaign again on realistic offline and it is working!

Worked for me, maybe it's something to do with the patch or whatever.

Got it on first try :s

I heard of this glitch but didn't know of any solution at the time, so I did my first playthrough in one sitting on Realistic. It unlocked.

Official, from Ubisoft: 
 
"Hi Guys,

We’re working hard on eliminating the bug that is preventing people from getting the “Realistic Difficulty” achievement for Splinter Cell Conviction (Complete Single Player Story on Realistic Difficulty). 

While we can’t discuss timing right now we can confirm that this issue will be fixed in the next title update for SCC. 

The fix will automatically detect if you’ve already finished the game on Realistic difficulty and award you with the missing achievement. This means you will not need to play through the single-player campaign a second time after the update comes out, it’ll all be handled automatically.

We apologise for any inconvenience this may have caused and thank you for your patience while we work to resolve this issue.  " 
 
http://forums.ubi.com/eve/forums/a/tpc/f/5271091065/m/7661093458

I hope they fix it soon before I have to bring it back. I don't want to have played Realistic for nothing.

I got the game the day after it came out, it had a title update, I installed that and then installed it to my HDD. Played through it from start to finish on Realistic and got the achievement.
 
I hope you guys get the fix soon, I know how rough it can be to put in all that work and not have anything to show for it. Not that realistic is super hard, just has a few tedious spots.

 Probably the fix fixes more than necessary 'cause I got this achievement after entering the menu for the very first time, without even playing once...