According to the demo, Siri can supposedly understand free form speech. "Tell me what the weather will be like in Boston on Thursday." is supposed to work and if you follow up with "How about Houston?", it is supposed to deliver Thursday's weather information for Houston.
If that is true, then that is indeed quite a feat.
The next step would be to allow text input into Siri. (which it does not currently have)
Let's face it, there are times when it would be inconvenient, rude, dorky, etc.. to talk to your phone in public.
Imagine typing "What time am I free on Thurday?" and having the application a) know that you are talking about next Thursday. b) understand that you are not looking for the first free slot (12:01 AM) c) ask you if you want to book a meeting for the time slot d) ask you who you want to invite to the meeting.

Yes, you can go to your calendar and hunt an peck but typing out a sentence is much more natural.

That would be revolutionary.