Road Shifters, Mountain Rear Derailleur? I have a road/cyclocross/commuter frame that is currently spec'd with road components - all Shimano 105, with an Ultegra rear derailleur.  

The cassette (10 spd) tops out at 27 teeth, and the rear derailleur has a very short cage compared to Deore/XT/XTR I've looked at.  If I wanted to use a cassette with more teeth, 32 or 36 - I was wondering if there'd be any issue with swapping out the current Ultegra for a 10 spd Deore/XT/etc cassette &amp; derailleur but still using the 105 STI?  Both shifters &amp; derailleur would be 10 spd, but I don't know if say cable pull is the same?
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.