I ran across this post from Mr. Phil Haack this morning when doing some ASP.NET MVC research.
I was messing around with my routes and one was not working properly and the technique described by Mr. Haack hooks in a catchall to prevent ugly asp.net error page saying it can't find the route.
We're on an older CTP of asp.net mvc, but when pulling in the 4 C# files into my solution, it worked flawlessly...now, onto solving the problem!
cheers
jk