From what I gather, I think most people talk about that in their SOP. And for those applications that have 'field of interest' sections, people indicate it there too. I don't think it's anything "official", really. As far as I can tell none of the schools hold you to what you write (or what you select on the forms). I think it may affect how they view your application though. From a Cosmic Variance blog post:
Is it true that the standards are different for theorists and experimenters?
Typically, yes, although it might be different from place to place. Because a lot of undergrads haven’t been exposed to a wide range of physics research, a large number of them want to be Richard Feynman or Stephen Hawking or Ed Witten. Which is great, since we need more people like that. But even more, we need really good experimenters. Generally the ratio of applicants to available slots is appreciably larger for theorists than for experimenters, and schools do take this into account. Also, of course, the standards are a little different: GRE’s count more for prospective theorists, and research experience counts more for prospective experimenters. And let’s be honest: many schools will accept more prospective theorists than they can possible find advisors for, in the hopes of steering them into experiment once they arrive.
So should I claim to be interested in experiment, even if I’m not?
No. Think about it: given that schools already tend to accept more students who want to do theory than they can take care of, what are your chances of getting a good advisor if you sneak into a department under false pretenses and have to compete with others who came in with better preparation? It makes much more sense to go someplace where they really want you for who you are, and work hard to flourish once you get there.
Do I need to know exactly what I will specialize in?
Not really, although in certain circumstances it can help. Professors like to know that someone is interested in their own area of research, and might push a little harder to accept someone whose interest overlaps with their work; on the other hand, most people understand that you don’t know everything after three and a half years of being an undergraduate, and it can take time to choose a specialization. In particular, at most American physics departments (unlike other countries and some other disciplines), it is generally not expected that you need to know ahead of time who your advisor will be when you arrive, or which “group” you will work in.
Here's a link to the full article, it's quite good: http://blogs.discovermagazine.com/cosmi ... te-school/
I'm in the same boat as you though, I don't really know what area I want to go into. For most of my SOP's I just listed a few fields I might be interested in and a few faculty members whose projects I liked, and said I'm generally interested in theory.