Details are scarce at this point, but MSNBC.com reports:
BREAKING NEWS: Mubarak to step down as Egypt's leader, Suleiman to take over, sources tell NBC
Update: NBC's Richard Engel says NBC has been told of the news by two separate sources "close to the president that he will step down tonight and make a statement." (Keep in mind that "tonight" is local time, not U.S. time.) He said the news was just filtering through the crowd. Engel also said he expected that the Egyptian military would soon make a statement on the transfer of power. You can watch live coverage on Al Jazeera English here.
Update: Al Jazeera English reports that jubilation is spreading through Tahrir Square and Alexandria as rumors of Mubarak stepping down filter through the protest crowds.
Update: Wael Ghonim tweets "Mission Accomplished" but follows up with a cautionary note: "wait and see."
Update: AJ English reports an Egyptian cabinet official says Mubarak's decision will come within hours. Various networks are reporting CIA chief Leon Panetta says there is "strong likelihood" of Mubarak's departure. NYT reports "Egypt’s armed forces on Thursday announced that they had begun to take "necessary measures to protect the nation and support the legitimate demands of the people,” a step that suggested the military intends to take a commanding role in administrating the strife-torn nation."