Microsoft has released Windows .NET Server RC1 to the beta testers. Thanks NaTaN.

The following products are available:

Standard Server
Web Server
Enterprise Server
Enterprise Server Checked (debug) version for developers
Enterprise Server 64 bit Edition
Enterprise Server 64 bit Edition Checked (debug) version for developers.

Registered beta testers can download RC1 from the Windows Beta Website

Not a beta tester? Check out the Windows .NET Server Customer Preview Program