The only technical limitation I can think of is the one which exists server-side. Siri has been having troubles from time-to-time with just iPhone 4S owners polling it. Imagine if all iPhone 4 (and even 3GS) owners were trying to use it too.
Do you care about that when you use Twitter or Gmail? Instead for Apple, after paying a shitload of money, you are super interested in how hard is scaling for them?!
There is no limit to my shock at how people are mentally gamed by Apple.
I have to believe that Apple has the engineering talent to scale something like this up. It's a pretty straight forward problem, and it's not like they don't have enough cash to double their server farm.
