Using PBX In a Flash based on Asterisk 1.8, I was eventually able to configure SRTP support and achieve encrypted audio calls between my Asterisk exchange and a Bria softphone running on my iPhone 4. Audo quality is fine. The best guide to the process I found was this one from Larry Sampas.
Things seem to have become easier, and in Asterisk 11 these simpler instructions seem to work well: https://wiki.asterisk.org/wiki/display/AST/Secure+Calling+Tutorial