I dont think it was on purpose... check this image out:
Right after it happened... she covered up.
From MSNBC:
Timberlake said he did not intend to expose Jackson’s breast.
“I am sorry that anyone was offended by the wardrobe malfunction during the halftime performance of the Super Bowl,” Timberlake said in a statement. “It was not intentional and is regrettable.”
I bet he was trying to spin her around to face him and as he grabbed to turn her, the outfit she was wearing had a seperate boob peice that ripped off.