I’m not sure how true this is but Spoiler TV posted a press release that says that John will guest star on 30 Rock on the 10th:

“30 ROCK”

(X) “DOUBLE-EDGED SWORD”

02/10/2011 (10:00PM – 10:30PM) (Thursday) : BOTH JACK’S AND LIZ’S ROMANTIC GETAWAY PLANS ARE FOILED. ELIZABETH BANKS, JOHN CHO AND MATT DAMON GUEST STAR.

Jack and Avery (guest star Elizabeth Banks) go to Canada for a final romantic getaway before they become parents, but when Avery goes into labor, they do everything in their power to not give birth to a Canadian. Meanwhile, Liz’s romantic getaway with Carol (guest star Matt Damon) is ruined when their plane is stuck on the tarmac. Tracy is besieged with the unwanted responsibility of being an EGOT winner.

SOURCE