Battle.net to grab the PC beta client for StarCraft 2! (It appears the Mac client is not available yet). Of course, you must have previously volunteered for the beta at Blizzard's site, so if this sentence is news to you, then ... sorry. You're what scientists call out of luck.
The client weighs in at 1.64GB and will take a while to download, so have a crossword handy. See you out there!
Update: Blizzard has revealed the minimum system requirements for the beta. Check 'em out past the break.