Well, this gets back to the never ending "discussion" on mods and warranty issues. Theres one going on now where some folks think a warranty is no big deal, and to go ahead and do what you want with mods.
Theres always a good chance that your dealership will still work with you on this repair; you'll find out when you talk to them about it if this problem persists.
What I would do is to head over to a parts store, have them do a free scan, and have them clear the code. Most automotive parts stores offer this. If the fault comes back, you'll need to repair it. Your code is part of the emmisions section. The cap (or in our Mustangs case, the seal in the neck) keeps the fumes from getting into the air we breath. The fumes get routed to the charcol canister, then eventually to the intake were they are burned. This does not rob any HP, so it's a good, clean emmisions thing. It's not engine damaging, so don't worry about that.
Also, never "top off" the fuel when fueling; when the fuel kicks off, stop, don't try to squeeze in any more fuel, as the overflow can (and eventually will) damage the charcol canaster. They are over a hundred bucks a pop. The over fueled fuel sloshes around when you drive, and gets into the charcol canister.
In your case, it sounds like the filler neck seal. Hopefully it was just a one time deal. See if you can clear the code, then drive on, and hopefully it will not set itself again.
