Since the beta test period that included 30 days of free service ended several weeks ago, many had been using their own completely legitimate AT&T SIM cards with active accounts in the Streak. Up until yesterday, this worked with no problem.
According to several tweets (here and here) and various posts on the Pocketables Forum, MoDaCo, and CNET, however, AT&T is now blocking the Streak from using its voice and data networks. Some people report receiving a text message from AT&T indicating that the device is not authorized or approved for use before the carrier pulled the plug. Tales of all the beta unit IMEIs being blacklisted, lines being suspended, and the possibility of never being able to use the device on AT&T even after it's officially released are also being shared like ghost stories around a campfire.
I don't have my beta device anymore to confirm any of this (I only have the UK version now), but you may want to hold off on picking up a used Streak on eBay or Craigslist unless you don't mind using it solely on WiFi or T-Mobile EDGE (assuming the latter still works).