Just my guess that once Apple sorts out usage of Siri that it will be expanded to other countries, more languages, and more devices. Hence the current "beta" label. I don't think it's just a hardware issue.
I'm sure Apple was hammered enough on the first day with the influx of iPhone 4S buyers testing out their new toy, if the iPhone 4, iPad 2 and possibly more were all trying to access the Siri servers as well, the whole system would have just died.
My guess is that Apple will continue to build up their servers and work on the Siri software, and then take it out of beta and release it worldwide and on more devices.