According to Brad Paisley's tweets he will make a guest appearance in today's South Park Episode 1607, called "Cartman Finds Love." The episode will air tonight, April 25, at 10/9pm CT on Comedy Central.
He was pretty excited in his tweets - tweeting yesterday "You'll laugh. You'll cry. You may shart. This wednesday, on a very special episode of @southpark" and then sharing his character cartoon picture (see above).
He spent Sunday on the set @SouthPark and posted a picture with the main characters sitting with him on the couch.
Update: In the episode (1607), Brad sings part of "The National Anthem" before a basketball game, between the Denver Nuggets and the LA Clippers. Later he joins South Park character Cartman on the "Megatron" in singing "I Swear", the 1994 hit popularized by John Michael Montgomery and the pop group "All-4-One". Here are a couple of snippets from the Episode "Cartman Finds Love". The whole episode can be seen here.