There will always be a market for a used iphone. The prices will just change once the new one is announced and then when it becomes available. I'm guessing that the new one will have stronger unlocking prevention measures, so the old model will retain some value for a while with the unlocking and jailbreaking crowds.
You still have some time though. The iphone will most likely be announced at wwdc tomorrow, but won't be available for purchase until the end of the week. AT&T has sent a memo telling employees about a promotion starting the 15th or 17th, so the sales won't start until then.