According to their twitter feed, RadioShack (oh, excuse me, The Shack) is doing a trade-in program. If you are eligible for an upgrade any time in 2010 (and I believe this includes those that AT&T bumped up by 6 months) then you can trade in your working iPhone 3GS (good condition only) and get $200 off iPhone 4. Those with iPhone 3GS can get $100 off the iPhone 4.
And those that aren't eligible until 2011, can still get a discount off of the new price.
And those that aren't eligible until 2011, can still get a discount off of the new price.