True, but if a file were actually open, the deletion would have no effect. As a neat trick, you can be downloading a file, say, using wget. Then move the file that's being downloaded, or even delete it. The download will continue as if nothing happened. Reason being that on *nix systems, a file descriptor is used, and as long as that is open, the file isn't really deleted. It's even possible to recover deleted files that are open this way.
And undelete depends on the underlying filesystem, journaling, etc.