A constant risk with a combo bike-bus commute is a bus rolling up with the rack already full. A couple of times a year that's okay; I can wait 30min for the next bus, or just lock up my bike nearby (no, the driver is not going to wait; I just ride ahead to a stop where it's safe to leave the bike during the day or overnight--I'm faster than the bus in town so have time to lock it up) and get on the bus.
The other day I just missed one bus, and when the next bus came half an hour later, it had a full bike rack. I said okay, it's a nice day; me and my ipod can wait it out. But then then next bus was also full, even though it was a 3-bike rack. And it was the start of a four day weekend for me, so not a good time to leave the bike locked up. Finally got on the third bus--that's 90 minutes waiting for a *%#!@*& bus, almost three hours to get home. Had I known, I might have camped out in my office overnight.