Data Are or Data Is?

Language Log, in a discussion of "usage fetishes," takes on the question of "data are" versus "data is":

... Copy editors are meant to be gnomes working invisibly below decks to ensure that the engine of prose runs smoothly. They shouldn’t obtrude themselves conspicuously into the middle of a clause, so that the reader has to break off his attention to the writer’s argument and do a little mental stutter-step before he can remark to himself, “Oh, I see—it’s that data-must-be-plural business.” Copy-editors desirous of such notice should try another trade...

I’m not going to hash over all the arguments about the singularity of data, which has been hashed over at some length, to put it mildly. ... My own view is that there are contexts where it’s okay to treat data as a plural, but none in which you can’t treat it as a singular—and that contrary to what many “reasonable” usage writers counsel, this isn't simply a matter of “style and personal preference.” ...