Let me start by saying that I've never actually used a bus bike rack, but I was wondering about the very same thing (potential for theft) just the other day.

It appears that most of you who say 'no concern' come from smaller municipalities with (presumably?) less crowded buses. Here, it is not unusual for a bus to be packed, such that I don't know if one could even see the bike while riding said bus (probably standing, near the back). And I don't know that the drivers would have any sense of whose bike it was (if they cared... I'm willing to bet that use of the racks is 'at your own risk' for the most part). Having said that, I am not aware of any thefts from the bus racks.

I tend to agree with the lock it to itself/immobilize it idea -- that will deter target of opportunity theft, which is probably the main concern. I can't imagine any 'real' thief riding the bus to scan for decent bikes -- there are too many better targets (like rail stations).