Anyone who buys a 3GS who already owns a 3G is a schmuck. What are you getting extra? a megapixel and a compass. Most of the improvements are in the software, which is available for the original 3G too.
Speaking of which, I am considering getting rid of my HTC Diamond now that the iPhone has push notifications, finally shit can run in the background.. kind of. Looking at a 3G on ebay with a cracked shell, I can replace it myself so it will be cheap as chips. I'm not a big fancy high flyer who needs 32gb on a phone, I only have like 6gb of music on my computer anyway, heck I'm still using a Philips HDD-100 mp3 player from 2003 in my car...