I don't know of any iPhone pre-paid plans.

I have an Android phone through Virgin Mobile with no contract (because I hate them, even though I keep my phones forever). They have several plans that range from $35-$55 a month. $35 gets you 300 minutes per month, with unlimited data and messaging; $45 is 1200 minutes and the same data and messaging; $55 is unlimited everything. It has 3G wireless, and of course WiFi and Bluetooth capability. It's an LG Optimus Elite (I think). The phone was around $150 and they gave me a $35 credit toward the first month's service.
"Maybe Lucy's right. Of all the Charlie Browns in the world, you're the Charlie Browniest."--Linus, A Charlie Brown Christmas
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My fotki: http://public.fotki.com/nynaeve77/
Password: orphanannie