I would acutally be slightly suprised if they didn't open with On the Backs of Angels, to be honest. I think the clean guitar opening of OTBOA would send chills down my spine, and get me totally jacked for the show. I definitely want to see Bridges in the Sky, however it's currently my favorite off the album, so if that was played first, although I would be happy, I'd have a feeling as though I'd already shot my wad.
Also as someone else said, OTBOA was our first exposure to the new era of DT, so it seems like it's naturally suited for that "first" spot.