My advice to you is to wait for the jailbreak for 4.0 to be released, then jailbreak & unlock and then sell. Your resell value will be much higher than if you were not selling it unlocked. I was able to sell my iPhone 3G last year for $400 unlocked. More than enough to cover the expense of the 32 GB 3GS that I bought that I'm also planning to sell in order to buy the iPhone 4. And since my 3GS still has the early bootrom that doesn't require a tethered jailbreak, I think I'll be able to sell it pretty fast again. Although this time, I'm hoping to get at least $450 for it.