Missed this a few weeks ago, but the Microsoft.com team have published performance data on their experiences with IIS 7.

All found here: http://blogs.technet.com/mscom/archive/2008/06/09/microsoft-com-operations-performance-analysis-of-iis-7-0-windows-server-2008.aspx

Bottom line... 31% more requests per second than IIS 6 with a slight CPU increase.