3G to 3GS? That's why I think we'll get a iPhone 4S with the iOS5 (and possible subtle improvements as shown on the CDMA version's metallic band) being shown off as their next big thing. At the time of the 3GS release, I'm sure Apple was waiting for the A8 for mass market profit. You could say the 3G and 3GS had a combined 2 year shelf life being pretty much the same hardware.
If you look at Apple's behaviour, it's quite predictable. The iPhone 4 is pretty much a very well rounded product that could do the same thing quite happily and last for 2 years. This time round, we have a CDMA version (and possibly white version) to take us through to an announcement in April 2012. Quite a crafty move to boost sales in 2011.
If sales are still up, why create a better product to destroy when the parts of the iPhone 4 are probably coming down in price month by month wholesale and they're making good profits? What could they add to a new iPhone in 2011 that isnt already on the iPhone 4? A time-machine?
I hate when I see people refer to the 3G to 3GS update as "minor". It was a significant update, and the hardware actually changed more on that upgrade than it did from 3GS to iPhone 4. It was leaps and bounds faster than the 3G. The CPU was upgraded, GPU upgraded, and internal internal RAM doubled. The only thing that didn't change between the 3G to 3GS was the external look and screen resolution. Maybe it's because it "looked" the same is why people thought it wasn't a significant upgrade.