Although this is based on nothing more than conjecture, i suspect that Siri will be available for the ipad 2 as a future update. In the keynote it was described as beta and that more updates etc would arrive in time. Perhaps Siri for ipad 2 will be one of those update.
It makes sense that it isn't already on the ipad, if it were included in the standard ios5 then it would have been in all the betas and would have subsequently stolen apples thunder. It obviously needed to be a differenentiating feature for the iphone 4S and wouldnt make sense to have had to release code early to developers. To just bolt it on to the firmware build for ipad 2 would not be smooth if you were to do it quickly in time for the GM. Even if they wanted to they couldnt add it to 5.0 for that reason.
In my opinion Siri is something that will be added to the ipad 2 and macs in time, just like facetime was after being announced for the iphone 4 (another new selling feature at the time). It reminds me a lot of the speech systems included in OSX builds of old where you held Esc and could ask it to do things like open itunes (which only worked periodically for me), but was included all the way back in Jaguar if my memory serves. Makes sense to use the technology in more than just the 4S, once it has launched and become established of course.