I just started using MU on IIS7. I ran into a problem with the wp-admin/includes/misc.php file. The XPATH statment was looking for an exact match of 'wordpress' when it should have used a starts-with expression.
The fix is simple.
file: wp-admin/includes/misc.php
replace: [@name=\'wordpress\']
with: [starts-with(@name, \'wordpress\')]
Hope that helps anyone with the same problem.