OK...
Like most people have said, it's not that we were fooled, it's what they chose to show. The idea of "April fooling" us is great, I agree. Also, if this episode had been shown any other time, it still wouldn't have been funny. T & P are funny in the context of an SP episode, but I think making a whole episode about them is like making a whole movie from a Saturday Night Live skit. Of course, that could have been the point.
I was not as much interested in who Cartman's father was as I was in how they were going to handle it. Therefore, my disappointment is in the fact that I expected some great humor.
All this being said, I will continue to watch the show. My hope is that when the new season starts and we do find out who the father is, they will return to form and present it in a hilarious way.
Just my opinion.