Maureen Dowd’s op-ed column in the NYT yesterday was headed
The Great Game Imposter
and later references to the Afghan man who passed himself off as a top Taliban commander used the spelling IMPOSTER. The day before, the headline in the news section went
Taliban Leader in Secret Talks Was an Impostor
and this front-page story used the spelling IMPOSTOR throughout.
The -ER spelling has appeared on Language Log, most notably in the title (and body) of a posting by Mark Liberman on 7/18/08:
Ranking fields by the difficulty of imposter detection (link)
(with comments addressing the spelling).
The facts are these: the -OR spelling is older, but the -ER spelling has been gaining on it, to the point where most current dictionaries give the -ER spelling as an alternative; both spellings are found in great numbers; but some people still consider the -ER spelling to be a mistake.
It was a bit of surprise to find the New York Times, which generally tries hard to enforce One Right Way, especially in mechanical matters, willing to let Dowd (or her editor) have the -ER spelling, and even to carry it over to the head.