http://mymercurialproject.codeplex.com
Read our announcement: http://blogs.msdn.com/codeplex/archive/2010/01/22/codeplex-now-supporting-native-mercurial.aspx
Check out our step-by-step walkthrough: http://blogs.msdn.com/codeplex/archive/2010/01/22/using-mercurial-on-codeplex.aspx
More info coming soon!