If you have a BlizzCon key from 2008, you can now register it at http://blizzcon.com/beta

Please note that the key on the back of the BlizzCon card is NOT a StarCraft II 'key'. When you opt in on the above site, your email will be added to a distribution list for a guaranteed StarCraft II beta key. When time comes, an email containing a StarCraft II beta key will be mailed out to those who registered with their BlizzCon key. At that point, players will be able to enter their StarCraft II beta key at their Battle.net account management page, allowing them the ability to download the StarCraft II beta client.

Reminder:

Many people have been asking where they can get beta keys for StarCraft II. Here are a few of the opportunities out there right now, and more will come as we approach closer to beta.

"Vespene Laughs" StarCraft II Comic Contest (reoccurring monthly)
"Vespene Laughs" Monthly Comic Contest http://forums.battle.net/thread.html?topicId=10697505926&sid=3000

Blizzard Theme Park Contest (Ends for North America March 30th, end dates differ for each region, check with the regional site)
This is a great opportunity to get creative, especially since we'll be giving out a pair of beta keys to each category winner. Go Go Go!
Win Beta Keys - Blizzard Theme Park Contest http://forums.battle.net/thread.html?topicId=15230879667&sid=3000

Warcraft III The Frozen Throne Tournament Season Winners 2009
The top 20 eligible participants at the end of each region’s invitational ladder will receive a StarCraft II beta key.
http://www.battle.net/tournaments/2009/faq.shtml

Upcoming Opportunities to look out for:
Beta Opt-in
BlizzCast Contests/Events
Future Find Raynor Screenshots (will be announced when it starts, unfortunately does not include previous finds)
StarCraft II Trivia
StarCraft II Fan Site Program
and more...