Seems to me that if Matt, Trey, and enough people at Comedy Central mentioned that Cartman's father would be revealed on April 1st, then maybe he/she/it really was. But everybody is so busy bitchin' and complainin' about the T&P episode they overlooked the fact that somewhere during the episode it MAY have really been revealed. For instance:
1) While T&P were watching South Park, Cartman says something to the effect "he's my dad?" So myabe something in that little scene gave it away. Not necessarily sound, like Mephesto saying it, but someone in the background or something written on the pad...
2) Maybe the father is sticking out somewhere in the background, like on the streets of Iran/Iraq (I know, what the hell's the difference) or at the game. Certainly any South Park character present in the T&P episode would be a good candidate for Eric's father...
3) Seems like everyone of those Canadians had their initials emblazoned on their outfits, maybe there is a name or initials spelled out somewhere that ID's the father. Especially the football game, there are a lot of letters in the stands...