I too have been looking to do a 50, and the ones I've considered in the fall are
SlickRock 50 in Moab-
Run completely on dirt, rock, and even sand, between Arches National Park and Canyonlands National Park, single loop trail with spires, arches, towering monuments, slickrock, sand dunes, river, and a view of the Rocky Mountains in the distance.
10-8-11
http://www.runmoab.com/Home_Page.php
But what might suit you best of all would be the Devil Mtn 50, Pagosa Springs Colo.
Run through some of the most remote and pristine old growth forest in the US, 100% off road, 80% singletrack, 8,300' of gain in the 50 mile, topping out at just over 9,800ft, beautiful fall color, crisp Mtn air.
10-1-11
http://www.joingecko.org/info.asp?uid=310
I personally am Interested in the
Heartland 50 in Cassoday Kansas-
Through some of the last remaining tallgrass prairie in the US. The openness of the course presents challenges you won't face in other races, and for 2011 is run as a night race, starting at 6pm. The Moon will be 92.3% Illuminated, rising at 5:11pm and setting at 4:22am. Read the Info Booklet for more, uh, info. It MAY be a day race again in 2012. Have to wait and see.
10-8-11
Race Info booklet-
http://www.ksultrarunners.org/hl50booklet11.pdf
http://www.ksultrarunners.org/
Best of Luck!