I could have probably told you this without doing all of the research, but Loopt asked around anyway, and found out that Siri was the iPhone 4S' most exciting new feature for consumers. Sure, that camera is really impressive, and that processor will come in handy when running new games and apps. But 45% of those polled say Siri is the killer app for the new iPhone. And that makes sense -- not only is Siri flashy and new (and available only on the iPhone 4S, for now), but it's pretty much the marquee feature for the device according to Apple's marketing.
And since that feature was only available on the iPhone 4S, then who, do you think, were the most frequent buyers of that new device? Turns out, according to Loopt's fairly informal data, that it was actually iPhone 4 owners. Personally, I usually go for a "leapfrog" system of hardware upgrades (and since I already have an iPhone 4, that leaves me out of this round), but that's probably not the case with most iPhone 4S buyers -- some people just need to have the latest and greatest, so according to this data, most day-one iPhone 4S buyers were standing in line with their iPhone 4s already.
Interesting. This information is all based on "Loopt Qs," which are apparently social questionnaires put on by the mobile social network, so it seems like these answers are self-selected and probably not really representative of anyone but the folks who use Loopt. But nevertheless, the insight that Siri is driving iPhone 4S adoption seems legit enough.