I had this happen to me and just got it fixed about 3 weeks ago. I figured out what was going on from the old bbs and took it to the dealer to fix it under my original 3yr/36k warranty. So I don't know what the additional warranty coverages is on this problem, but it is a known issue that ford never bothered to fix on even the newer cars. (mine is a 2000, the TSB is from '98, you'd think they'd have fixed it in between but no.) Hope you don't have to pay for it, I remember reading it was a real cheap part + $700 - $900 in labor to drop the gas tank and exhaust to replace the cheap part.
What happens is there is a valve on the fuel filler neck that I believe swells (possibly because of ethanol in gasoline) and doesn't allow the pressure out of the tank.
This only seemed to show up when it got hot outside, it also may have been affected by brand of gasoline because they have different percentages of ethanol added to them.
It is more inconvienent than dangerous, but of course having gas come shooting out of the tank while you fill it is always somewhat bad.
TSB 98-15-3 is the TSB on this particular problem, I just gave that number directly to the dealer with a brief explaination of what was happening and they actually fixed it on the first try
Hope this helps
-CtrAltDel
