HK version of iPhone 3G 8GB and iPhone 3Gs 16GB/32GB seem the answer for those who want to buy it in advance before available in your country. And yes, i am talkjing about Pay And Go (not contract) Why?
All pricing here;
HK http://twitter.com/iphonethai/statuses/2558158906
AUS http://twitter.com/iphonethai/statuses/2439709188
CA http://twitter.com/iphonethai/status/2391138773
USA http://twitter.com/iphonethai/status/2389688058
UK http://talk.totocyber.com/2009/06/iphone-3g-s-uk-pricing.html
HK version is the answer, even bits pay more but worth on 'unlock'. US, CA, AUS are cheap but require resident id+TAX etc. AUS can get unlock w/ fee. However, all locked' iPhone 3Gs can be able to unlock now (Firmware 3.0) but you have to keep follow the news on every updates in the future, else just stop at the one that work perfectly.
All price may vary, you just have to check the price of iPhone suppliers in each country in the future.