Scott Guthrie announced on his blog that ASP.NET AJAX has finally released after lots of hard work from his team and lots of input from developers. They have also released the source. Scott says it best, so I'll point you to his blog post....
Also, check the ASP.NET AJAX 1.0 Resources and “gotchas“ that Scott listed in this additional post. Make sure you make note of migration issues from RC to RTM.