Sadly no one's reading past the OP's post.If anyone thinks these files are sitting on some room of shiny Xserves in Cupertino, think again. As already mentioned in this thread, it's all hosted with Akamai, distributed across hundreds of plain old rack servers, probably running Linux.
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. 😛
A Gold Master has to go to the phone networks for testing seven days before distribution to the public - not so with the Touch firmware, so that could go out on time.
It's probably because simultaneous launch of 3G + 2.0 + MobileMe = fingers badly burned, so there's probably been some sort of edict from on high that they don't launch more than 2 major products on the same day any more in case they need to field support calls due to issues.