Jim Dalrymple’s iPhone 4S review. Regarding Siri:
Siri is not your typical voice recognition technology. You don’t dictate to Siri, you interact and have a conversation with Siri. It’s difficult to explain how good Siri is, but you’ll find out soon enough.
Maybe Siri being iPhone 4S-only truly is just a sales tactic. Maybe Apple is confident that Siri is so great and clever that it will actually compel a decent amount of people to upgrade who weren’t otherwise planning on it. However, on the other hand, for some reason that doesn’t sound very Apple-y to me.