3 PM Eastern, on his facebook page. Folks were originally supposed to have to “like” his page in order to be able to see the special message, but evidently somebody leaked to NBC:

Former Minnesota Gov. Tim Pawlenty (R) will announce that he has formed an exploratory committee to consider running for president in 2012, a senior adviser confirmed to NBC News.

Pawlenty will make the announcement today at 3:00 pm ET on Facebook. The message will be accessible to Facebook users who “like” the former Minnesota governor’s page on the social networking website.

Update: If you did the “like” thing you can now see Pawlenty’s video ad announcing his exploratory committee. It’s a great video.