When I lost my sidekick a while ago I got a virgin prepaid/pay as you go phone while I waited for my new one. It's just a simple flip phone and you only pay for the minutes you need. The phone itself is like $13 and they have minute cards from $10 and up.
The service was good--I had service in basements and the top platform at the train stations... But I use my phone way too much, so pay as you go was costing me a lot more in those couple weeks than my real phone.