If I play, for example, Plants vs Zombies or Toki Tori on Steam when I am offline, can I earn achievements? Are these re-synchronized when Steam returns online?
Well - it's some years later and.. it still seems to depend on the game the most. I play recently almost exclusively offline and connect gaming machine to Internet once every few weeks. E.g. in 'Botanicula' - with almost all achievements gathered on the way - I have only the second 1/2 of them on Steam (some of the missing ones being impossible to miss). :) 'Bejeweled 3' seems to sync all achievements perfectly (with their progress also!) on next online session. For 'Plants vs. Zombies' I have only 1 ('Home Lawn Security') on Steam while I have around 3/5 of them in game itself.
There are many threads around the web regarding those games AND achievements, but the common conclusion is: Some games just don't sync them properly and that's it. :(
Since recently, you do get achievements in Offline mode, if you have opened the game in Online mode before, or viewed its details.
Typically yes you CAN earn achievements offline for most Steam games. One major downside is that upon your next connection to Steam Online, all of those achievements earned offline will "unlock" at the same moment, resulting in identical timestamps for the cluster of offline play. This can make you appear as a cheater on achievement trackers.