“we have decided remove the initial 2.5 million limit on the public beta for the next two weeks (thru January 24th). During that time you will have access to the beta even if the download number exceeds the 2.5 million unit limit.”
More information on the decision at http://windowsteamblog.com/blogs/windows7/archive/2009/01/10/here-s-where-we-stand.aspx
P.S. This is only important if you don’t have an MSDN or Technet subscription.