How secure are bikes at races?

I've never seen or heard of a bike getting stolen at a race.

Some are more secure than others. Some races let people besides the athletes go into transition, and that could potentially invite more problems just because there are more people. Some races check your race number against the one on your bike and some don't.
I haven't personally heard of someones bike getting stolen, but I'm sure it has happened on occasion.

Like stated already it has happened and I've read about one or two occassions but I've not seen it first hand. The smaller the events, the more relaxed security is (if there is any). I'm doing one in a few weeks where there is absolutely no security (did it last year so I know), but I really don't worry about it too much.

I think it's too big of a risk for the task. If you think about it, the more expensive the bike the more likely someone (you or buddy), will be looking it out for it.

Let me say this, I'd feel bad for the guy that tried to steal a bike and got caught. Can you imagine the beat down put on that sucker?