If (to simplify the maths) we make the assumptions that:
100000 + 1000000 + 10000000 + 100000000 = --------- 111100000 (base 62)A figure which is far too large to usefully undertake an exhaustive search with current technologies. Don't forget, however, that passwords CAN be made up with even more characters then this; you can use
However, it's still MUCH more efficient for a cracker to get a copy of "Crack", break into ANY account on the system (you only need one), log onto the machine, and spoof his way up to root priviledges via operating systems holes.
Take comfort from these figures. If you can slam the door in the face of a potential crackers with a robust password file, you have sealed most of the major avenues of attack immediately.