Based on selling experience, if the iPhone4 can't be unlocked (the 3GS couldn't be unlocked when it first came out but the 3G could), then the 3GS prices go right back up.
Exactly. This is why I will not be selling my 3GS until my iPhone 4 is in hand. Then I will pay back my account and have a little profit at the end of the day. The unlock is typically more important that the jailbreak but you have to be broken to be unlocked. People want to throw their t-mobile sims in and use the edge network or use their existing AT&T accounts without extending their contracts.