I was just looking for an answer to this. I played through Fallout 3 with a few mods (hair & body stuff mostly and the weight limit increaser) and I still earned achievements. I was worried that this game using Steam instead of that horrible Games For Windows Live junk might have changed that somehow. I want to do a run through on hardcore but with some cosmetic changes as well as increased weight allowance and don't want to find out that I missed out on that Hardcore achievement because of it.