"The beta database has been reset to help test a launch-condition environment. This includes a wipe of characters and identifiers, friends lists, profiles, league placement, and ladder ranks," explained a Blizzard representative.
It is unfortunate that players will have to re-populate their friend lists, but it will be nice to be able to play the ladder placement matches again within a larger population. As such, ladder placement should be much more accurate after a few days.
Last night, I placed into the Platinum League, winning four of my five placement matches. I expect to drop back down to Silver shortly.