[quote name='pittpizza']
Anyway to answer your question, it may be too late for whoever you swapped out to sub in liara for the noveria mission. You have to do almost the entire game (including side quests) with the same character to get the ally ach. For liara, i got the ach right before the last boss fight of the game and I used her on almost every single mission there was.
My advice if you're concerned about the ally ach and don't want to redo Noveria: do every single sidequest there is with the two ally's you want the ach for. I don't mean most, I mean EVERY single one. You may still get it. You don't have to worry about losing an ally (without spoiling anythign

as there are ways around it.[/QUOTE]
I switched Wrex out for Liara during Noveria and did EVERY sidequest and I still didn't get the Wrex achievement. What sucks is that there really isn't any difference bringing Liara along even though the other characters tell you to - I think Benezia says one thing about her and then otherwise it is exactly the same. On my second playthrough I got Ash *really* early (before even doing Virmire) and then wrapped up Garrus during Virmire, and I didn't even do all the sidequests. It looks like you need 15 story missions and 35 sidequests to get the ally achievements. Someone here said 45 before but that definitely isn't the case as I had exactly 35 when I got the Ash one. Unless it gets easier with the New Game+ or Hardcore difficulty? That actually makes sense as I would think I had 35 with Wrex the first time and didn't get it. But GameFAQs (just saw your post Callandor) is definitely wrong as far as my situation (not sure if it is the New Game+ or the Hardcore that is changing it though).
My problem is that I'm trying to hit level 60 on this second playthrough. I got to 50 at the end of my first one and then started up the New Game +. You progress pretty quickly until 56 or so and then you need a buttload of exp to go up a level. I'm not quite to 59 yet and I only have the endgame left. Vermire put me 2/3 the way from 58 to 59 and I got out and hand-killed everything (no shooting from the Mako). I'm also playing on Hardcore although that doesn't seem quite so hard any more (it was very tough in the beginning and not so bad since then).
Anyway, when I'm done with this playthrough I will have 3 of the 6 ally achs (d'oh!) but will probably have to play a bit of a third game to hit level 60. We'll see. Then I still have some of the weapon ones (assault/sniper) and a few tech/biotic ones. I'll wait for more of the DLC to appear (more than just this one) and then start a completely new game for those. I'm getting sick of my same character

.
One thing I read in this month's GI - they said ME 2 will look at your save file and modify the universe based on decisions you made in your playthough. Here's a potential problem - on my original game I played for Paragon points and would want that to be how ME 2 continues. But on my New Game + with the same character I went through getting the Renegade stuff done. How will it decide what to do in that case? I guess I could delete the game saves after I restarted (once I wrap up level 60).