Someone has devised a clever yet altogether impractical way of getting 3G speeds on iPhone by pairing it with a Windows Mobile Smartphone equipped with HSDPA. Now, rather than go into details I'll simply refer you to the the website to see and judge for yourself. And I won't be putting this process to the test because my sole Windows Mobile device, a newly purchased AT&T Tilt, is being returned due to the fact it is an utter piece of bovine anal excrement.
For those who crave adventure and just love cramming radioactive gadgets in your pants, go for it. I won't stop you.