After 8 years it was time to change the .NET logo. The new logo stand for consistency, robustness and great user experiences.
The design look like a "wave" with a suggestion of the letter 'N' that will become recognizable over time as shorthand for the .NET brand name.
From:
http://blogs.msdn.com/msmossyblog/archive/2008/10/25/embrace-the-new-net-logo.aspx