UPDATE - 5/12@7:13AM: The episode is now available online at this link along with some more details. The Rush reference comes at about the 6-minute mark. Rush is performing at a candlelight vigil for the princess where they play a version of Candle in the Wind but with a distinctive Rush sound. Geddy sings the line, ... it seems to me you lived your life like a flower breaking wind... and then farts. Alex then shoots himself and Geddy breaks down crying... :)
The prince of Canada is about to take a princess and thousands have gathered for the royal wedding. Ike Broflovski is obsessed with it. When they're about to get married, the princess is abducted. All Canadians must find her, and Ike answers the call to arms.
At one point in the episode they mention the Canadian rock group Rush then cut to the band performing for an audience. While playing, Alex ends up shooting himself in the head because he is so distraught from the kidnapping. Coincidentally Royal Pudding happens to be the 212th episode of South Park. Thanks to cartmancop for the heads up.
South Park creators Trey Parker and Matt Stone are big Rush fans and friends of the band. They famously created the Lil' Rush concert video that was shown just prior to Rush's performance of Tom Sawyer on the Snakes & Arrows tour.