Terrific idea!
I rode the Illinois Diabetes Tour de Cure in 2004 (my first big ride) and it was great. The best thing was the wonderful support and organization (and the big dinner afterwards!). I finished feeling like I had climbed Mount Everest! It really proved to me that I could take longer rides and not just survive but set a bar to beat next time.
It's also wonderful to actually do something physical and help a cause at the same time. It's frustrating that you can't help those you love when they are sick or suffering - rides are such a good way to work out those frustrations. Of course, I'm sure it would be more efficient to just give money to the associations and let them save the expense of organizing a ride but that wouldn't be nearly as therapeutic!
I guess the worst thing about any big ride like that (aside from lousy rest stops and poor route marking) is if you are riding by yourself and you don't manage to hook up with any ride buddy along the way. I always ride better with some distraction/motivation from someone else.
I'm going for the MS 150 this summer and I hope to find someone in the area to ride with. So, no dirt to dish yet!