A friend did this to his honda civic @ 60mph. He did it later to verify his findings.
It puts alot of strain on it but you should be ok. He only kept the car for another 6 months but it didn't have any problems.

Most automatic transmissions are 'fluid drive'. the engine and transmission don't actually connect. The engine drives a 'propeller' (yes,like in a boat) and there's another one in extreemly close proximity that's attached to the transmission. that's why the car can idle in first gear while stopped.
Some newer transmissions have 'lockup' units in them that actually lock the tranny to the engine. With that setup I assume that it would theoretically wrench the engine right out of the car ; )

HTH