I know Sapphire Longbows has done a carbon "I" Beam reinforced 3 piece riser, and the talented young bowyer says it makes a big difference...not just in terms of speed, and stiffness, but also being dead in hand and quiet.
Sid Border uses Carbon in his risers. I believe O.L uses carbon in his risers as well.
I've head some say that the best bang for you buck with carbon in trad bows IS in the riser. Especially with the difficulty in getting a qualily prepreg carbon lamination source. Even some of the most connected, most experienced carbon bowyers get an occasional bad batch.