Probably best just to get a new O-ring.

Sometimes they swell up a bit from the gasoline, and then they just won't cooperate.
Use grease on it for sure.

Trust me, it was lubed up like a mo-fo.  I'm still not having any success, so I hope to find a new o-ring on Monday.  Thanks for the help!

New o-ring procured and gas cap assembly is installed.  Everything is once again right with the world! 

The fuel filter had indeed been changed at the 12K service, so all I can figure is that the tech didn't tighten the hose clamp enough on the fuel pump.  Or, perhaps it wasn't right from the factory and finally worked intself loose.  Not a big deal really.

I appreciate the help!

Excellent work, Always good to see how people track down faults.
I love to learn this sort of stuff.