Steam needs to keep offline achievements and playtime and sync them once you get online. I mean come on, EVEN
Games for Windows Live does this for achievements.
Pretty sure I read a post recently about this actually happening already. Something about having to have played it connected already and then when you play offline it synchs (including achievements) when you connect again. Edit: reading a bit more it seems to be a bit random. For some games it does, for others it doesn't.
Having *cough* 'cheated' some of my achievements by using a savefile I know the achievements unlock when loading a savefile as well, which doesn't matter then whether that was done offline or online, since they all loaded as soon as the savefile was loaded.
To be honest, if there's anything I'd really want, it's a way to reset achievements. (not for the games mentioned above but rather some of the RGG's I bought and the GF played first. Would be nice to be able to do it myself with achievements as well)