Road Shifters, Mountain Rear Derailleur?
Won't work. Dyna-Sys uses a different pull ratio than Shimano road components, and the cassette uses a slightly different spacing as well. You could use a SRAM cassette, but that still leaves you with the derailleur issue. They do make 105 and Ultegra long cage derailleurs, however. If you've got a double up front, especially a double with proper cyclocross gearing (tigher ratio than standard or compact road), the long cage road derailleur should be more than adequate.