Dude, you should be able to make the upgrade. Your derailleurs will handle it no probs. You will need a narrower chain though b/c the spacing between the cogs has been reduced from the 9 speed. 12-27 is pretty standard fare and should get you up and over all but the steepest inclines.
Here is what Shimano says about the ten speed compatibility:
Shimano Ultegra CS-6600 10 Speed Cassette
Ultegra® steps into the 10-speed world with its 6600 cassette. Patented Shimano® ramp and tooth profiles for effortless shifting. Compatible with Ultegra® and Dura-Ace® 10 speed cassette bodies as well as 9-speed 105®, Ultegra®, Dura-ace® and most other 9 speed cassette bodies. Requires Shimano super-narrow 10 speed chain. Available in the following sizes:
11/23
12/23 (12, 13, 14, 15, 16, 17, 18, 19, 21, 23)
12/25 (12, 13, 14, 15, 16, 17, 19, 21, 23, 25)
12/27 (12, 13, 14, 15, 16, 17, 19, 21, 24, 27)
Weight 235 Grams (12/23).
I'm surprised I didn't find that Shimano info when i was on thier site looking at the cassette itself!?? Sounds good though cause i really didn't want to swap out more components than i had to. Yesterday i was going up a pretty steep ass hill and thought I could really use a 27 in the back...burnt out my legs getting to the top. But i like the mid-range gears in there since i'm more of a spinner than a grinder. Thanks Simon! Beers on me next time i see you for your better research skills.
Go 27 dude.. a no brainer!!!
Definately a 27. Going to line the pockets with more gold then do the upgrade. Curious to see what bikes shops will say about upgrading...good time to weed out the bad shops vs good ones.