you can also try to hold the ibook with the slot facing down and hitting the eject button then... try to shake it a little bit when the drive tries to eject the disc.
another (more hardcore) way is to stick a credit card into the drive when the cd is spinning and try to stop the disc by pressing down on it with the credit card... that should trigger the emergency eject.
@madmax: holding the mouse button on startup only is good when you have software sided problems taht prevent the disc from coming out - like an unreadable disc but the OS tries to read it and crashes while doing so - this wont trigger the emergency eject - its only a normal eject...