No he isn't. Read it from his own website change.gov:

from Open for Questions: Response | Change.gov: The Obama-Biden Transition Team
Q: "Will you consider legalizing marijuana so that the government can regulate it, tax it, put age limits on it, and create millions of new jobs and create a billion dollar industry right here in the U.S.?" S. Man, Denton

A: President-elect Obama is not in favor of the legalization of marijuana.