http://www.quora.com/What-proportion-of-all-iPhone-owners-us...
It's a genuine question, even though it sounds rhetorical. The same applies to people still using pre-3GS devices.
I'm working on the assumption that these people simply won't spend (much) money on apps, and that the effort involved in supporting older devices and OSs won't ever be repaid. If it's going to take you even one day, probably not worth bothering. iOS development is a crapshoot enough, without your spending time trying to support a wider range of devices. The longer you spend coding support for older devices, the smaller the percentage of the market they will be by the time you release.
(This is particularly true for OpenGL stuff, but I imagine it goes for Cocoa as well.)
Consider European markets where costs to upgrade iPhone 3G to 3GS/4G are much more than $200.
After being live for 45 days, it has around 50,000 users total at this point. Of those 50,000 users, less than 2% (817) were using pre-4.0 devices.
At that point, it was easy to convince them to abandon 3.x support for the next version, which will greatly simplify a lot things for us. I'm anxious to see how quickly 5.0 is adopted once it hits.