I spotted a member in a DigitalPoint Forums thread with a level of certainty that Microsoft will be announcing at adTech San Francisco today that they are opening up ContentAds (also may be known as PubCenter) as a public beta. ContentAds is Microsoft's competing Google AdSense product.
An open beta means that virtually anyone should be able to sign up for the product and test it against Google AdSense. It is also rumored that Microsoft will set up a companion blog for the ContentAds (PubCenter) today, April 22nd.
In March 2008, Microsoft began accepting beta users via a sign up form. Since then, it has remained fairly quiet except for some comparisons made by beta users between ContentAds & AdSense. ContentAds began being tested in June 2007 by a few publishers and some really early tests in October 2006. The earliest rumors of this product came in December 2005 and then again in February 2006.
Hopefully the big beta launch is today.
Forum discussion at DigitalPoint Forums.
Update: Microsoft confirms this and adds more details.