There's a report somewhere on this thread, I think, that a bus driver saw him try to go in, but he left when they tried to search his pack. Then, according to the driver, he went to the bench, sat down, and started taking something out of the pack. If true, it would seem he changed his plans and decided to smuggle the bomb in on his person, and it went off as he tried to transfer it from the pack.
Remember that an American convert appeared in the last videotape warning. Does anyone think that was just an accident or a novel idea? To me, it was a glaring statement from the folks at AQ that attacks would now come from such American recruits. It might also have been a message to said recruits that their time had come to be the front line.
That makes sense to me. And it would have been fairly easy to get something that fits into a backpack underneath a baggy shirt, and into the stadium. But a bus driver who could see the bench could not have seen the stadium entrance - buildings in between. Maybe he just saw him head that way, and then return? Although, FWIW, the bench (south oval) area is not on the way to the stadium from his apartment (if the address I've read is correct.)