Not regarding spoilers, I've just heard that when you beat BS the game is done, done like dinner, done like... well done. is that true? I do have the Goty ediiton on ps3.

If i'm recalling right it does the opposite to the game, before BS you weren't allowed to play after finishing the MQ. BS raises the level cap and picks up where the original MQ left off.

this is correct, broken steel extends the main quest and adds 10 levels and a few new perks

but after beating BS, can you continue to play or does it just end?

No. After the Broken Steel story, the game will still resume. With the Broken Steel DLC, Fallout 3 goes on forever.