The following postpay devices are offered at a promotional price when activated on Prepaid service: Samsung U340, LG VX5400, LG VX5550, Chocolate, 2 Alias.
There are no touch phones that will work on pre paid phones. Online they only show four phones but there are more call customer service to find out which ones can be changed to pre paid. I know Samsung Intensity and LG Accord are of pre pays, and I think Chocolate and Alias the old one not Alias 2. I'll keep looking for more information for you.
Don't forget the LG VX5600. Those are currently being sold at different retail outlets prepackaged for use with prepay service, and I've heard that LG is a good quality brand. Also the VX5600 gets excellent standby and talk time per charge.