According to a press release sent out last week, several pieces of South Park video content were meant to be discounted as well, though it doesn't appear as if the discounts have been applied yet. We've contacted Microsoft for clarification on the issue.
Update: Episodes from the "Cult of Cartman" and "Matt and Trey's Top 10" South Park video collections are now discounted to 80 MS Points each in the Zune Video Marketplace.
Xbox.com: Add South Park XBLA to your Xbox 360 download queue ($5.00)
Xbox.com: Add 'Poker Face' (South Park Version)" to your Xbox 360 download queue ($1.00)