The same will be true of the Samsung Galaxy S2 - those users won't be queuing up to replace it after 12 months (or if they are it's a bad sign rather than a good one).
You're not wrong about it as a factor, just saying that analysts should get this and figure it in.
The 4S is looking mighty tempting though, and I was going to get it unlocked. (Because I hate being locked to three year contracts, stupid Canada.)
You won't be disappointed, the 4S is so much faster at everything. And Siri is amazing. The only downside is that I have started looking for the 'Siri button' on my iMac.
Glad to hear that the upgrade is worth it though!
However, iPhone 3GS users for the most part have had the phone 24 to 12 months, and if you are eligible for an upgrade with AT&T, it seems you pay the iphone subsidy each month (by my estimates, around $50 per month) regardless of whether you upgrade or not. So it makes sense to upgrade and sell the new phone on ebay if you want to keep your 3GS.
I was also basing this off of the Sprint iPhone costing roughly the same as the ATT iPhone, and Virgin Mobile supposedly using the Sprint network.