Also, too (an affectation I've adopted for the specific purpose of keeping the Sarah Palin fiasco fresh in people's minds: The mapping linked in the post immediately previous to mine doesn't show any regionalism to my marginally practiced eye. The ratios appear evenly distributed across population centers in the continental US, with the correct form still dominating for the present. (Thank whatever powers there are for small favors.)

Recent conversations on the subject dominated by Pacific Northwesterners and myself (New England born and raised and a hardcore "by accident" purist) revealed an interesting wrinkle: there are actually adults who use both, preferring to apply "on accident" to themselves and "by accident" to others. Further probing was rewarded only with the vaguest sense of a difference, but imputing intent does seem to be in play, i.e., one knows one's own intent and says "on accident" contrary to "on purpose," but with others one can only make the assumption that it was "by accident" based on reaction rather than intent. (Personally, I don't buy it, but my prescriptive criticism isn't going to make much headway with adults set in their linguistic ways.)