I'm currently without any kind of mobile phone at all. I'm looking, somewhat seriously, at Ting (ting.com). They're unique in the US in that they don't have plans at all. If you use a primary feature of the service (voice, messages, data) then you pay for what you use and that's it. It's not quite pay as you go since payments are divided by usage blocks but it's the closest thing that I've seen in the US to date.