This is no different than previous years.
You can still sell your 3GS, and since it doesn't come tied to a contract (with a $300 ETF), you should be able to get a good amount for it.
The only problem is craigslist/ebay/etc. become absolutely flooded with previous gen iPhones right around the launch of a new one. People willing to go a bit lower on the price can get theirs sold faster.
That's what I'll probably do.