In case you are on the road to an iOS 4 update (and be careful), you might want to double check your work. Remember, first you have to update iTunes to 9.2, which will then allow you to download the iOS 4 update.
After I updated iTunes, I went in to make some changes to what I had synced and take off some podcasts that I don't listen to and video I don't really watch. After making changes and clicking apply, I got this error:
"iPhone cannot be synced. A duplicate filename was specified."
A bit of searching found other people having this issue, and the clear explanation at Apple Toolbox saved the day. As it turns out, the culprit is iToner from Ambrosia Software. I used it once a long time ago, and then I deleted the application without using my handy dandy AppZapper. As a result, it left behind a little file in my iTunes Plug-ins folder that I didn't realize was there.
I removed the file, and as if by magic, everything started working again. However, if FedEx isn't lying and my iPhone actually does show up tomorrow, it was a bit silly of me to jump through all of these hoops to run iOS 4 on my 3G ... for one day.