I think I may have worked it out.
We aren't getting 2.1 later. We're getting 2.1.1. The iPod touch owners have had 2.1 early for debugging purposes. Apple know how important 2.1 is to iPhone users, and so they want to iron out any bugs they find. (And judging by the iPod touch forums on apple.com, there's still plenty of bugs in there!)
Remember that Apple also removed several bits of functionality surrounding Push in the 2.1 betas, so these functions weren't tested fully out in the field. 2.1 could be the biggest beta programme in the world ever!
Cue 2.1.1 becoming available later for both iPhone and iPod touch.
Of course, it's just speculation.