I'm just making sure.... i'm waiting for fedex to show up.... I had my iPhone 3G jailbroken too and I don't know if that wold have affected the backup.
The guy at the Apple Store allowed me to purchase the 3GS for my wife, activate it, swap Sims with my 3G, and reactivate both. So, the 3G went to my wife, I have the 3GS, and I restored the 3GS with one of my previous 3G backups.