Here is a quick hack for upgrading to an iphone 4 and getting some additional compensation (selling your 'old' iphone 3gs for $450) for relegating yourself to a life of dropped calls with AT&T for an additional 2 years. Basically, this hack requires that you are:
1) eligible for the discounted iphone 4 upgrade price of $199 for the 16gb and $299 for the 32gb... and i feel bad for those people who preordered but are still waiting to get their iphone 4... considering you can now walk into an apple store and buy one before yours will arrive.
2) within your applecare warranty... or at least close (2 weeks off Apple will normally still cut you some slack)
The system is quite simple, make a genius bar appointment at your nearest apple store.
1) meet with your rep, convince them to exchange your phone (dropped calls, erratic phone behavior etc)... most of the time it helps if you've done a recent phone restore to prove that you've really taken time to solve this problem.
2) I also had them replace the headphones with a new set (they're staticy or they cause your phone to jump into voice control randomly... as mine really did)
3) leave the original plastic on the replacement phone they're giving you
Once you have your replacement iphone 3gs, wait in line to buy your new iphone 4. If they give you any gruff about having just got a replacement tell them you're giving that one to your mom... everyone knows family is the most important (well, tied w/ breakfast).
When you get home, find your iphone 3gs box & assemble the new headphones (if you didn't exchange these just take the ones our of your iphone 4 box) along with the new usb cable & the new wall plug from the iphone 4 box. Take a nice crispy picture of the phone & accessories with the improved camera on the iphone 4 & write a quick description of your "new iphone 3gs" (don't forget to mention that this replacement iphone 3gs has an additional 90 days applecare protection on it) & upload that shit to craigslist or ebay.
I managed to sell mine for $450 cash within 2 hours of posting my ad on CL... you might be able to get some more money out of it by unlocking & posting it to ebay but that seemed like more work than I wanted to do at the time.
Best of luck,
Grant
