Emporia, Virginia. It's actually an independent city rather than a county (a historical quirk of Virginia), and Christians as a whole still outnumber Muslims, but there the Muslim congregation is larger than any one denomination of Christians.

http://www.thearda.com/rcms2010/r/c/51/rcms2010_51595_county...

I've been trying to dig through their data to answer your question, but I don't have SPSS/STATA, and the SPSS viewer I downloaded was pretty ineffective. It's here if you want to try and find it:

http://www.thearda.com/Archive/Files/Downloads/RCMSCY10_DL2....