Nah
I have no plans on selling my 16GB 3GS as I just bought it.
My old POS phone died recently and on reading that 3GS phones would support OS4 when it comes out, was very happy to commit for 2 years. It wasn't long ago that the 3GS was the duck's nuts and everyone was going ga-ga about how wonderful it was. Now, people seem to be going ga-ga about what the new 4G will be able to do with its 5 megapixel camera, forward facing video camera and so on but for me the 3GS is just fine.
So - no sale!