Do you think anybody still on 3.x will actually spend money on random apps for their phone, that aren't backed by some massive marketing campaign and/or network effects? (Obviously, if you're working on such an app, my thinking might not apply ;)

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.)