There used to be a cap of 40 on the amount of debuffs you could have on a creature at one time. Well, according to Daelo on the O-Boards, that cap has been increased with Wrath of the Lich King.

We changed how the game handles debuffs (ie negative state effects on targets) on creatures with Wrath of the Lich King. The old hard cap of 40 debuffs on a target no longer effectively exists. You can now apply way more debuffs to a target without them dropping off before their duration expires. The default WoW UI will not normally display all these debuffs, but they really are still there!

Of course, Daelo does not confirm whether there is a new hard cap for debuffs. From the use of the words "way more," we can probably assume there is some sort of cap.