Last night, he wanted to have a private meeting with Hillary Clinton. So he simply got the press on his airplane, had it take off, and waited until they were in the air before having them informed he wasn't on the plane.
From the NYT:
"Sunlen Miller, who covers the Obama campaign for ABC News, filed an urgent dispatch via Blackberry to report that the senator had abruptly changed plans and had given the slip to those who were traveling with him all day. “I sent it as the wheels were going up,” Ms. Miller said of her message, recounting the agitation and confusion among her fellow travelers as the 757 lifted off.
It wasn’t until after the plane landed in Chicago — sans the presidential candidate — that Mr. Gibbs confirmed a meeting had taken place between the rivals. Details? None given."