The short answer is "No."
The longer answer is, of course, more complicated.
Do you see the additional microphone hole in the top of the iPhone 4S? Specialized circuitry exists in the A5 processor that allows it to use that second microphone for advanced noise cancellation. This is what makes Siri so good at understanding your voice.
People who demoed the dictation on the iPad said that it was hit and miss in loud environments like the demo room, because it couldn't discern the speaker's voice from the background noise.
Additionally, Siri is just a lot more useful on a phone (which will always have a data connection) than on the iPad which may not (in the case of a WiFi only model). Plus, you're more likely to have your phone with you than not.
Personally, I don't see how Siri would be that useful on the iPad, but I understand why people want it. The iPhone seems like a much more natural fit. I'd rather they work on some way to wirelessly exchange data between an iPhone and iPad, similar to how HP demoed it with the Touchpad and the Pre.