Allo Martin, was gonna text you last night after seeing the M BMW on top gear, hint hint

Yes it will be the same on 3G, and yes you will need to re-jailbreak.

Have a read here mate, instructions and downloads are there as well.

I think you are still on o2, therefore dont need to unlock, so heres your instructions:

Locked iPhone 3G
If you are using your iPhone with one carrier and have no interest in the possibility of an iPhone 3G unlock in the near future then just restore or upgrade to 2.2 using iTunes and use QuickPwn to Jailbreak and add Cydia and Installer.

HTH