OTOH, if Apple had killed the 3GS this time around, rather than keeping it around for another year, that probably would have been better for devs since they wouldn't have needed to support that old slow platform.
I wouldn't call the 3GS slow. The 3G was slow, and represented a development pain in which we would do optimizations to suit the device. 3GS is plenty fast, and the iPhone4 and iPad2 are smoking hot rods.