I stumbled across this guy's solution for a fender light mount which I thought was pretty clever:
http://www.krabach.info/bike_light/aux_flasher.html
Based upon that, I came up with my own solution. The PB Cascadia has a small gap between the fender and mudflap to insert a shim of plastic that's firm enough to support a Super Flash. Note the hole cut into the plastic for the belt clip. The light doesn't move around at all either horitzontally or vertically, and it cannot sink any lower into the mudflap as it appears to be sealed. The light isn't perfectly perpendicular to the road, but it's close:
http://i167.photobucket.com/albums/u...07-14-2008.jpg
http://i167.photobucket.com/albums/u...07-14-2008.jpg
http://i167.photobucket.com/albums/u...07-14-2008.jpg
http://i167.photobucket.com/albums/u...07-14-2008.jpg
http://i167.photobucket.com/albums/u...07-14-2008.jpg
http://i167.photobucket.com/albums/u...07-14-2008.jpg