here is a more complete statement of the "rule", to which there are probably still exceptions:
The general rule is that when one of the suffixes "-ed", "-ing", "-er", and "-est" is applied to a word ending in one consonant preceded by exactly one vowel, the consonant is doubled if and only if the word's final syllable is stressed: "omitted" but "edited"; "preferred" but "offered". Americans obey the stress rule when the final consonant is "l": "repelled" but "traveled". Britons double "l" regardless of stress: "repelled", "travelled".
this is from alt.usage.english FAQ
...isn't that marvelous?
