I just did an online chat with a VZW rep, here it is:
Please hold for a Verizon Wireless sales representative to assist you with your order. Thank you for your patience.
You are now chatting with 'Kiara'
Kiara: Hello. Thank you for visiting our chat service. May I help you with your order today?
You: I was wondering if 4G phones like the Droid Bionic are eligible for the Prepaid Smartphone plans???
Kiara: I do apologize for the inconvenience, although the Bionic is not able to be pre paid
You: Are their any 4G phones available for the Prepaid plans?
You: And/or can you tell me if there are any plans to offer any???
Kiara: I do apologize although at this time there are not
You: Ok, thank you for your help
So, looks like you can't, at this time, use a 4G phone on their prepaid plans.
I currently have a Nexus S on T-mobile's $30 month prepaid plan (100 mins, unl. text, unl. data*throttled after 5GB), and I love it but I don't use my phone much for calls... I plan on getting the Galaxy Nexus and using it on this plan once it's available.