Prepaid plans or cards for Android PHONES?
You can get the LG Optimus V from Virgin Mobil for $150 (total) and pay $25/month for unlimited data and texts and 300 minutes/month of talk time. There are three drawbacks. (1) The screen is a bit small. (2) The internal memory is only about 200MB, so you are limited as the number of apps you can have (even moving apps to the SD card). (3) Virgin Mobile uses the Sprint network which seems to be more limited than say Verizon. 

Later this month (I think) Virgin Mobile will also off the Motorola Triumph with 2GB internal memory and a larger screen and a faster processor and a front facing camera (which the Optimus doesn't have, so maybe that's four drawbacks). I plan to switch to the new phone as soon as I can.