Thanks to sn0wbreeze you can upgrade your iPhone to the latest version of the iOS, the number 5 and still unlock your iPhone to use it with any carrier using your old gevey SIM card.
Sn0wbreeze is needed to preserve your baseband, so go and download it here: http://ih8sn0w.com/
You will find that the procedure to unlock your iPhone is a bit diferente, I.e. you won't see the little bar in the carrier name area before dialing 112, instead you'll see a "no service" tag. You have to continue normally and it will work. Or at least it worked for me.