I don't know that the iPhone 3GS will see as much demand as the iPhone 3G over the first few months of its release, but as of now it's off to a strong start.
Note that it took 74 days for the first million iPhones to sell, 3 days for the first million iPhone 3Gs to sell and just another 3 days for the first million iPhone 3GSs to sell. That puts the early pace of the iPhone 3G and 3GS at a close tie.
So what do you think? Will the public continue to adopt the 3GS at the same rate as the 3G? The 3GS's hype certainly hasn't been as strong as its older brother, but maybe Apple's position in the mobile market has strengthened since then.
Shameless confession: As long as my iPhone has strong app support, I really don't care how many units sell. In fact, the less the better. Those Apple geeks are sooooo annoying. [Apple and Apple and Apple]