It’s probably ok to drive on for a while. The worst failure that I’ve read about included a spring coming through the side of the flywheel. That failure damaged the bell housing, carving a groove in addition to throwing grease.
http://www.e90post.com/forums/showthread.php?t=879563
On my car there was no damage but I didn’t want to risk it, so I decided to replace my clutch now.
You will most likely hear noise if damage is being done.