Help Please. No power to the motor right after the dealer replaced ignition switch

[yukon]

Well....the dealer went to get the truck this morning and (as Murphy's Law would dictate) it started for them. They drove it to the shop turned it off and when they went to start it....nothing (all electrics worked, engine didn't crank). After a couple of tries it started, they moved it in and hooked it up to the computer and found nothing. The truck they proceeded to start flawlessly every hour on the hour. It even started for me when I picked it up.
There was no charge ('yet' as the shop manager informed) as they couldn't find a problem, but its a little unnerving to know that my wife and two infant daughters could get stranded in a parking lot somewhere in minus 30 degree weather...
If anyone has any other suggestions please pass them along. And for the guys who already helped out thanks again....
dave