Great tip here on how to run IE7 BETA 1 without breaking IE6.
http://weblogs.asp.net/jgalloway/archive/2005/08/12/422335.aspx
Ian
*UPDATE*
This now causes an issue with the dec 05 updates for IE6, and is not supported:
http://blogs.msdn.com/ie/archive/2005/12/16/504864.aspx