I've read about six different people, just today, using the phrase "is in his head" to mean something like "has him rattled" or "is thinking one step ahead of him." I'm familiar with songs in one's head and voices in one's head, but I haven't heard the phrase used this way before. Does this come from a movie or something?

I'm not sure where I first heard it, but I think it stems somewhat from the related expression "Get out of my head!" It means roughly that the person it's said to is anticipating the sayer's thoughts too well. It's likely also related to the song connotation, in that a particular idea is stuck in your head, only in this context that idea looms over everything you think and colors your decisions--like an athlete trying to get past the last losing game, or a doctor trying not to repeat a fatal mistake, both of whom would be better served by clear thinking but are unable to get to that state.

I'd venture a guess the head you've been reading about is Obama's, and it's Palin or McCain in his head. I'd further guess this has a little to do with the infamous "lipstick on a pig" gaffe. (It's also been said that McCain, drawing on his training, is "inside Obama's OODA loop".)
Lummox -- I'll buy your hypothesis until someone proves otherwise! Funny you mentioned the example of an athlete; after posting this, it also occurred to me that this might be a phrase from the sports world.

Your guess about the head in question is correct. Though it's tempting to believe he did it deliberately, under the auspices of plausible deniability, personally I suspect he meant for Palin to be the lipstick and for McCain and/or the GOP to be the pig (which is still a little demeaning, but not nearly as much, IMHO) and he's just a schlemiel to his own schlimazel. But then, we live in a culture of "offensensitivity," as Berke Breathed put it (around two decades ago), and Obama of all people should be aware of that, seeing as how he's been its main beneficiary in recent times.

I've seen many references to the OODA loop meme as well (likely originated by some unsung Bill Whittle reader). It's interesting to see that both campaigns seem to be picking up tips from the blogs this season. E.g.: Sarah Palin's line about mayors vs. community organizers was a paraphrase of a comment on National Review Online, and right before his lipstick remarks, Obama gave a near-verbatim recital of a Toles editorial cartoon. And if you search Google News for obama lipstick pig, you'll see there were numerous uses of the phrase prior to Obama's -- many of which came out and directly stated that Palin was the pig to which they referred.

I'm not going to get cocky (Star Wars meme there) because there are still two months to go and we can still look forward to interesting times, what with October Surprises, and Chicagoans rising from the dead to vote. But it will be very interesting to see how the Obama campaign performs now that he has to ride without training wheels. (A meme to watch for!)

Some might deduce from the above that I am reading WAY too much political stuff. TO them I say: I don't have a problem! I can stop anytime I want!

[Edit: Oops, an editorial cartoon isn't a blog -- though I wouldn't be surprised if it came to the campaign's attention through one.]

[Edit 2: A Rachel Lucas commenter referred to it as "lipstickquiddick." Ha!]
I'm not sure specifically where it came from but I know its used a lot in sports and has been for a long time. The short definition is to get inside the mindset of your opponent to gain a strategic advantage. Most of the time it is part of a psych-out. You let your opponent know you know in an attempt to shake their confidence in their own actions.
Its very applicable to things like racing (getting inside the head of the other driver to predict their movements and improve your own).
You can't win a serious game of cricket without getting inside the opposing captains head. Coaches in your NFL would be in the same boat.

Freaking out and yelling 'get out of my head!' is also just good old fashion fun.
DarkView wrote:
I'm not sure specifically where it came from but I know its used a lot in sports and has been for a long time.

Mystery solved! Thanks DV!
Actually the sports analogy makes a lot of sense there. It's long been said that teams that play to win often win, but teams that play not to lose often lose. The specter of a potential loss can become a self-fulfilling prophecy very easily. Picturing a big defensive lineman pounding you into jelly could be all it takes to avoid confronting him with 100% of your strength and throwing a crucial block.