This can be done in IIS: http://weblogs.asp.net/joelvarty/archive/2009/03/23/force-ie7-compatibility-mode-in-ie8-with-iis-settings.aspx
Read the comments as well: Wednesday, April 01, 2009 8:57 AM by John Moore
A quick follow-up. This worked great for my site as long as I use the IE=EmulateIE7 value. Trying to use the IE=7 resulted in my site essentially hanging when run on IE8.