MG Siegler over at TechCrunch thinks that's about to change. In a big post on what we can expect from Apple at its big developers conference he drops this nugget:
There have also been whispers about Siri for iPad finally coming. Specifically, I’ve been led to believe it’s more of a UI issue than anything else. After all, Apple is using the technology for the Dictation functionality found on the new iPad. They’ve just been working on what Siri for iPad will look like, I’ve been led to believe.