Yep. My guess now is that they wanted to start the system off with a slow steady stream of users hitting the new Siri servers. (as with any beta service) Throwing tens of millions of iPhone 4 users as well would have been a disaster. (4S users overloaded the servers all by themselves) If they bring it to the 4 it'll be after the beta period. I wouldn't bet on them doing it at all though.

Looks like "slow" is going to be hard to do for Apple going forward.