Yesterday they started offering some plans for prepaid plans. I'm using the $1.50 unlimited day pass plan. I can access pretty much anything, even stuff that didn't work with the Sidekick plan, like e-mail and maps. Skype and Gtalk work too.

I have my APN set to internet2.voicestream.com. Then open a web page; any non-WAP page should work. You'll be rounted to a page that will let you buy the day pass. Once you go through the prompts and hit Accept, you should have working internet for the day.