Are you using Leopard? I can't get drives to eject right half the time.
its a bug, you cant eject a drive if you are using it or a different one, i had this issue when i was reinstalling all of my stuff i would be using a mounted image to install something, and i couldn't eject previous images until i wasn't using any of them
It about has to be a bug, glad to see I'm not the only one experiencing this issue. Hope Apple fixes it soon...