Just heard this piece of news on the Michael Kay Show. According to Kay, he'll begin serving his punishment immediately.
Unless the "Joba Rules" are amended even further (ie: not allowed to pitch on back-to-back days), this suspension essentially makes Chamberlain unavailable for the entire weekend series with the D-Rays because I would imagine Torre wants him available for two games (Mon and Wed) against the Mariners.
Update [2007-8-31 17:59:33 by anaconda]:
Here’s the official press release from MLB:
New York Yankees pitcher Joba Chamberlain has been suspended for two games and fined an undisclosed amount for his inappropriate actions during the top of the ninth inning of yesterday's game against the Boston Red Sox at Yankee Stadium. Bob Watson, Vice President of On-Field Operations for Major League Baseball, made the announcement.
Chamberlain will begin to serve his suspension tonight, when the Yankees host the Tampa Bay Devil Rays.