The only way to get an unlocked iPhone 4 is an official unlock. AT&T doesn't sell those. So you have to import from another country. Right now iPhone 4 is only available in USA, Germany, UK, Japan and France I think. As far as I know you can buy officially unlocked phones in UK. So that would be the only way right now.
If you have enough time to wait for a jailbreak and unlock, you can buy it of course. The advantage would be that right now you can make shsh backup of 4.0 firmware. Maybe you need that later for the first available unlocks. In a week or so there will be a firmware update, so you can never get the shsh file anymore after that release.
