Has a single person been caught and charged for using LimeWire?

Yes. The RIAA (Recording Industry of America Association) tracks and will prosecute IP addresses they find violating copyrighted material.

One example is at North Carolina State University, where in the Fall of 2007, 32 students received letters from the RIAA asking for $3000 to settle the charge of copyright infringement. The NC State students did not take kindly to this, and will battle the RIAA in court this coming year.