The memory difference is a pretty good reason. I was still using a 2G when 3.0 came out and it was never nearly as snappy again throughout all the 3.x releases

The memory difference is a pretty good reason. I was still using a 2G when 3.0 came out and it was never nearly as snappy again throughout all the 3.x releases I had on it. The 3G, with double the RAM, is a bit slow with iOS4 in spots. My understanding is the way memory management is designed in iOS Apple can't risk significantly decreasing the amount of memory available to third party apps without causing some big issues. I would imagine iOS4 apps will also have a bigger memory footprint than 2.x and 3.x apps as well. I'm sure someone will hack it onto the 2G so we can see for ourselves within a few weeks. I'm betting lots of sluggish operation and low memory warnings.