I don't know if Ziphone will unlock the iPhone 3G, and no, jailbreaking it doesn't unlock it, it just lets you run apps that Apple doesn't want in the App Store.
So, if you want an unlocked iPhone, search online and buy one that is factory unlocked. There are also some easy SIM card tricks that can unlock it too.
Just recently, the iPhone-Dev team have unlocked the iPhone 3G and will be releasing their whole thing on December 31, 2008.
So, if I were you I would wait until they release QuickPWN, which will jailbreak and unlock the iPhone 3G.
I just noticed this thread was from last month...sorry