It seems logical to consider costume design alongside art direction, since they go together a good bit of the time, and the occasions on which they haven't might be instructive.
Alice in Wonderland (Colleen Atwood)
I Am Love (Antonella Cannarozzi)
The King's Speech (Jenny Beavan)
The Tempest (Sandy Powell)
True Grit (Mary Zophres)
First thing to note: the correlation with a best picture nomination is way down from the art direction category. Nearly half the winners in the past 50 years were from secondary-level films...including those with only a few or no other nominations.
The second things is, voters seem to like to let their freak flag fly here. A certain dutiful drabness may get by under art direction, but when it comes to costumes, they like COLORS!! Consider 1974: Godfather II won art direction, and no one would have groused too much had the film won costumes, too...but Oscar voters instead went for the flashy flapper outfits of the otherwise-disliked Great Gatsby. Likewise, 1985 saw Out of Africa take art direction, and the various African garb on display would have made an inoffensive costume winner...but not when compared to the vivid battlefield hues of Ran, which took home the prize. 1992 -- Howards End vs. Bram Stoker's Dracula -- and 2008 -- Benjamin Button vis a vis The Duchess -- are further examples. About the only major exception I can think of is 1982, when the dread Gandhi swept the board, a result that makes Victor/Victoria fans (and even not-especial-fans like me) groan to this day.
So, where does that take us this year?
Complete flops like The Tempest have no chance. True Grit has drabness and little best picture buzz, so can be ruled out. And contemporary films win about once in 25 years, so the actually quite beautiful I Am Love is reduced to praying for a miracle.
The competition appears down to Helena Bonham Carter's two entries, Alice in Wonderland and The King's Speech. The latter is more loved by a factor of infinity, but what's the precedent for such non-eye-catching costumes, however in period, winning this award? Topsy Turvy, maybe -- though the onstage costumes probably had a lot to do with that winning. Chariots of Fire, the upset signaler of 1981? That at least had Alice Krige's frocks to carry it. No, the precedent is probably nearer Gandhi -- which is to say, if King's Speech is truly the steamroller many think (and that Attenborough's film was), it could win.
But there's more history behind n Alice in Wonderland win, because of its flash. It all comes down to whether people hate the film enough to deny it the prize.
I'm still thinking.