We've seen that fail to start scenario a good bit.  It seems to be about 50/50 switch and ignition lock.  The tail piece on the lock wears and won't turn the switch quite far enough if the switch is also worn.  We have replaced just the tailpiece on the locks and gotten the problem cured, but sometimes it's also the switch.  In short, it can be either or both.
Steve Sandlin, Registered Locksmith
Steve's Lock Shoppe
http://www.steveslockshop.com