It's pretty easy to disable single user mode on Apple hardware. Anyway, half the time that you might have time to get at single use mode, you probably also have time to just steal the damn box (as nihil pointed out).
MacOSX is pretty new, there are a lot of undiscovered holes. It has the advantage of newness, which means there aren't really many click-n-run exploits, but as those of us who read bugtraq know, it's far from perfect. From a pure security standpoint, I'd give OpenBSD higher ratings than MacOSX. So, I guess, 2nd most secure laptop ever :-)