What's the best way to set up my iPhone on a prepaid plan?

I’m about to procure an iPhone 3G, and I don’t want to sign a contract of any kind. I know there’s wireless plans out there for around 50 bucks a month for unlimited everything that are prepaid. So what’s the best plan? How do I go about getting that set up? If a prepaid plan offers unlimited data, does that work with an iphone? Does it depend on the sim card in the iphone at all? If anyone else has done this, I’d appreciate the insight. I’m in Denver, CO if that helps.