Matt Stone admitted that the writers of South Park did not see Inception when writing this episode and looked on the internet for material to work with. One of the things they borrowed was a CollegeHumor's own video about Inception, for which CollegeHumor called out South Park on.
The poster in the "regressed" Mr Mackey's bedroom says Space:1989 on it with a picture of an Eagle spaceship. This is clearly a reference to the Gerry Anderson TV show Space:1999
The title is a reference to Inception