Soma just announced that the beta period for VS2010 and .NET 4.0 has been extended and the release date is being pushed back a couple of weeks (no new date mentioned yet).
Also, in addition to the 2 beta’s that have been released there will be an additional public Release Candidate that everyone gets to play with 
Read more: http://blogs.msdn.com/somasegar/archive/2009/12/17/visual-studio-2010-and-net-framework-4-beta-period-extended.aspx