One would assume that with the launch of the 3Gs, there would be less demand than the launch of the 3G. Simply put, currently there is a much larger Apple subscriber base. Meaning, that most folks that wanted an Iphone already have one.
Secondly, you have to consider how the economy is going to impact potential sales. Consumers have less disposable income now, than 12-15 months ago.
Thirdly, I wouldn't be surprised if the decision to lower the 3G to $99 cannibalizes sales of the 3Gs. If people determine that applying the new update to the 3G gives them a phone they can live with, they may not feel the need to grab the 3Gs. Additionally, the $99 3Gs would appeal to those individuals whose disposable income has deteriorated.
Lastly, it would not surprise me if Apple "managed" the supply of the 3Gs after the initial launch. You know the old saying, "people want what they can't have". So if carefully control the amount of 3Gs phones at launch, restrict the supply of iphones for a couple of weeks, the media will eventually run with stories about how the 3Gs is sold out. If you are hear stories on how Apples third generation phone has sold out, and you are on the fence about buying an iphone, you may think to yourself "man it must really be a good phone. Maybe I need to get one".
Personally I wonder if after the first couple of weeks after launch, it becomes more challenging to find the 16 gig rather than the 32 gig. I'm sure there are a number of potential customers that would rather pay $199 to buy a new iphone rather than $299. Therefore you may see plenty of 32 gigs, while the 16 gigs may be harder to find.
Isn't rampant speculation fun??