Actually....I've had a lot of undergrad research in theory. Im not sure what you mean by "real" theoretical research, but if you mean graduate/publishable level, it is available to undergrads, as you said, tho I couldnt tell you to what extent. I had an REU in nuclear theory- I don't know know if you'd count this as "real," research, but I dont see why you wouldnt, since it was definitely graduate level, not to mention it had never been done before and was therefore genuine "research." Ive also done a couple years of AMO theory and we published a PRL, so this certainly would be "real" research in theory by any standard.
That being said, if you dont have research in theory and you want to do theory, I would think that youd still be ok, provided you have research in experiment or computation (which is, I guess, theory anyway) you can tie into your interests in theory in your personal statement. From what Ive gathered, if you don't have any research whatsoever, you might be in trouble, unless there are some extraordinary circumstances.
But don't let this get you down- first, research experience can be hard to come by. Second, Im just a college senior and don't know a whole lot about admissions, so please don't take my word as expert opinion....