Tom-
They used to have a distance race in North Carolina where they did this- Started all the cats after all the monos had left. The first time I did the race on my 18 square meter I beat the Committee boat (a huge Hatteras motoryacht) to the finish marker and just sort of sat there waiting for them. They didn't believe that I was actually in the race at first, but since I had passed every other boat in the race I had plenty of witnesses!
Having sailed in many "mixed" fleets I agree with almost everything said here- I always try to be gracious and stay out of their way- it helps to have sailed/skippered/crewed on some monos so you can relate to what they are going through and they're thinking. To me the biggest issue is the huge speed difference, especially in a puff- most mono guys are used to scanning slowly for approaching boats and they judge passing/crossing encounters based on their experience (which is usually other monos) and so can be extremely surprised when you just suddenly "show up" on their hip/quarter/whatever. I definitely "drive defensively" when sailing with monos- Never plan to get too close to them as they may suddenly tack just as you are about to pass to windward or gybe just are you are planning to go by.
I personally enjoy starting dead last, sailing through the entire fleet of monos (to weather upwind!) and finishing before any of them. The only downside is then waiting to start again. Agree with Mark about "horses for courses"- there is a "come all" race in Shreveport I have attended a number of times- in the typically light air the Thistles and MC scows have a clear advantage but one year the wind blew well and the top 5 boats were all cats - even on handicap!!

Kirt