There wasn't much of a gap between the 3G and 3GS. Devs recognized this and didn't create many 3GS only apps because it would cut their consumer base in half. It just wasn't worth creating 3GS only apps.
I expect this to change with the iPhone 4. There is a large gap between the 3GS and iPhone 4, with a ton of speed, resolution and other hardware features added. It also seems as if more consumers are upgrading/adopting this time around (judging from the preorders). Devs will take advantage of the iPhone 4 specific hardware this time around, so we should see more iPhone 4 only apps.