I went around 2:30 to wait in Philly last Sunday, and they didn't arrive until about 4:20, which is especially late. They didn't fly down from New York until Bono was done at the EDUN launch party. Otherwise, I probably would've been there at just the right time.
The most important thing that will make them decide to come back out after they've driven in is the behavior of the crowd. If people rush at their SUV's (all black Lincoln Navigators) or act stupid, they're not going to come out. As hard as it is, just stay calm! Even though they didn't have time to stop in Philly, Bono rolled down his window and I got a few pictures of him. His SUV was first, then one with Edge in it, then one with Larry and Adam.
Since they'll be in Boston for the rest of the week, I'd say your chances are going to be pretty good, but I bet there's going to be a lot of people, too. Good luck, and enjoy the show! I'm still scouring TM for tickets, but I don't think I'm going to get lucky. Oh well, I've got at least three more shows in the fall...