In his Twitter feed MuscleNerd shared the following information:
for *jailbreak* purposes 2.2.1 is fine for all including ipt2g. But it BREAKS the yellowsn0w soft *unlock* for iPhone 3G
it's not surprising that they fixed injection hole for yellowsn0w, it is surprising to see a "silent" update
the iPhone 2G baseband is untouched .. I *think* they've stopped all development of it :) last update for it was 8 months ago
It is quite likely that Apple released this update just to close the unlock hole used by the iPhone Dev-Team. We will keep you up to date with any further information released. Please DO NOT update.