Airlines don't advertise it, but we had 2 car seats delayed on a trip 2 years ago, turned out at least United and US Air have car seats available at most destinations if yours get delayed (or worst lost).

We regularly check our car seats (2-3 trips per year) and have not run into any problems other than the one delay. (funny thing is the seats were actually on our plane, but they assumed they needed to be brought up as gate checked and no one noticed for several hours)