You can learn more about Groening's appearance in this thread. You can also check out the preview/speculate thread for more details.From SNPP
Matt Groening guest voices as himself when Mrs. Krabappel calls off her wedding (after Principal Skinner starts having second thoughts himself) and runs off with another man (hint: "big fat geek" - hint #2: Matt's appearance is at a comic book convention)
The thread and poll will open immediately following the broadcast of the episode. You can also look forward to a repeat of "The President Wore Pearls" at 8:30/7:30c.