Does research experience in a different but related field help in PhD application? (Pure Math->TCS)
I'm not an expert, but, from what I know, your experience is quite valuable.  Even if you had computer science research experience, there's a good chance that what you would actually end up doing in a PhD program is far from the domain of your undergraduate research.  

If you're still worried, note that theoretical computer science is all mathematics.  I was an applied mathematics and statistics major and am currently taking a graduate course in approximation algorithms--it's literally a graduate course in theoretical side of applied mathematics.  Other theory courses in automata, computation, machine learning, etc are also all math courses, but in the computer science domain.  I actually think that your math/research background, if anything, is a huge plus.

What PhD programs are really interested in is whether or not you have done research in any discipline (preferably math/engineering for CS) and if you enjoyed it and showed an aptitude for it.  You're ahead of the game, don't worry.  On that note, if you know that you want to do TCS research, you could start now.