I have tested my brand new Mobily iPhone 3G with an STC SIM card and it worked.
AhmadT tested it, too and in the UAE Shufflegazine tested the Etisalat iPhone 3G and it’s unlocked as well.
So you can simply buy the iPhone 3G and switch the SIM card to the carrier of your choice. You will still be paying for a 12 month plan however you can put that SIM card in another phone if you want.
This will have an interesting side effect in Saudi Arabia as I am sure some people will buy the iPhone 3G from Mobily only to resell it at a higher price without the contract.
Update: The iPhone 3G in Saudi Arabia is officially unlocked by Apple.