Bushmaster puts out a Carbon-15-based 9mm AR. It is not known whether they use their own pattern, or if they're also based off of the Colt system.
Model 1 Sales puts out 9mm uppers and complete rifles. It is unknown which (if any) system they are compatible with, although it is likely that they're based off of the Colt system.
Olympic Arms puts out uppers and complete rifles. Olympic uses their own dedicated system.
Rock River Arms puts out uppers and complete rifles. Randall Rausch of ar15barrels.com states that, although he is not sure about the firing pin, "Barrels, bolts, recievers, mags and magazine adapter blocks all interchange" between the Colt and RRA systems.
9mm AR-15 Vendors - Uncertain Production Status
CMMG has been known to put out 9mm rifles and uppers. However, their website, as of 9/23/08, mentions 9mm products but doesn't list any. This info from apbrian112, of Calguns
Colt has put out complete 9mm rifles in the past, but I'm not sure if they are currently in production or not. I haven't been able to find any for sale on their website directly, but they may sell to dealers.