I've been watching this carefully. I bought the original iPhone in the US before they made you sign up for AT&T in the store, I'm English but these didn't sell them out of the US at the time but as the dollar was so low they were extremely cheap (for us). For several months I used it a rather nice iPod until a rather complex jailbreak and unlock came out later that year. From then on my new Nokia E90 was put in a draw and I became a proud iPhone owner. For many more months it remained unavailable outside the US and it became a show piece in meetings.
I didn't get the 3G, mainly because it remained un-hackable for some time but last month I was in line outside the London Apple store at 7:30am waiting to get my hands on a new 3GS. For the last few weeks I've been walking around with two iPhones, one old one with my Vodafone card in it and one new one with a pay-as-you-go (Â£10/month) O2 card in it.
Tonight I downloaded the Purplera1n (mac version), connected my 3GS to my Mac, backed it up and clicked on the "Make it Ra1n" button. A couple or re-boots later, some 5 minutes and I was the proud owner of a jailbroken iPhone 3GS. I downloaded Ultrasn0w on Cydia, installed it, rebooted and inserted my UK Vodafone SIM and it's now all working perfectly.
I wouldn't recommend doing this unless you really need to, I could have switched to O2 but I think they rip people off with their data prices (as do AT&T), I can get a full 7.2 meg HSDPA and UPA where I live on Vodafone compared to O2's rather slow 3G service. Although most people I know are using a hack to tether their 3GS on O2 I've been doing this on Vodafone for some ten years now starting with my trusty Psion and an RS232 link to my old Nokia phones, sadly that was still faster than today's data service on AT&T though in most of the US.
If you're adventurous or want to have a bit more flexibility over your provider then go for the jailbreak and unlock, I can verify that it works on the iPhone 3GS.