The whole point of the DNA tests was for Cartman to find out who his father was. Well, he found out last night. Just because we couldn't hear the answer doesn't mean anything. Cartman knows the answer, and I have a feeling the next new episode is going to pick up from there. Trey and Matt lived up to their end of the deal, without actually "revealing" anything and introducing new plot twists. I guess we'll see if I'm right in a few weeks...
Grant