It depends are where the lesions are.  I had an experience that was not
directly related to breasfeeding, but it was a way to let Mom hold her baby.
 She was HIV positive, with a massive outbreak of shingles across her right
trunk, below the breast.  It hadn't crusted over, so we covered it with a larg
e sheet of Op-Cite we got from the OR.  On top of that, we taped a chux,
plastic side out.  With this is place, and very strict handwashing, the mom
was able to hold her baby.

I work with a physcian (specialist in HIV+ Moms) who has neonatal
transmission rates of about 6%, and he doesn't let the moms breastfeed.  I
don't actually have any hard data on transmission rates via breastmilk, so if
anybody has something to share, I'd love to hear it.  It's hard to argue with
6% transmission, though.