Generally, how does a complimentary first class elite upgrade work for U.S. airlines, such as Delta, Continental, United, USAir and American?

For example, I am silver medallion on Delta. So 24 hours before my flight, i check-in, and if there is a first class seat available, then i am upgraded? always?

Is that true? If so, that means its first come first serve right?

What about other airlines I mentioned, is it the same?

Thank you-