There's a new beta opt-in procedure at Blizzard's recently revamped Battle.net that StarCraft II beta hopefuls should probably go check out.
Relatively quick and painless, the new beta opt-in procedure foregoes the usual uploading of DXDIAG files in favour of a more Blizzard-flavored method of obtaining your system specs. First log into or create a new Battle.net account and head to the beta profiles page. Then download the Blizzard system check utility, which quickly sorts through your computer's innards and uploads the data to Battle.net. Make sure you've got StarCraft II checked, click "Update Preferences", and you're done.
Now all you have to do is hope you get picked among the millions of others also opting in for the beta. Good luck!