Microsoft just released a new build of IIS7 and Windows 2008 Server which available for testing by Beta testers.
This build includes updates and fixes made since the Beta 3 release earlier this year, and also includes the previously announced IIS7 on Server Core configuration.
There is also a great article available describing the steps needed to get IIS7 started on Windows Server Core.
Currently there is no support for ASP.NET when using IIS7 on Windows Server Core because the .NET framework is not supported at all.
Keep that in mind when thinking about using this II7 option!