Until a few weeks ago, to enable TLS support in Asterisk you had to apply patches to the source code. As of version 1.6, Asterisk will have native TLS support for SIP transport, making it one of the few IP PBX systems out there that support this security feature.
Too many experts suggest encryption as a solution to the VoIP confidentiality issues, but in reality the number of PBX and IP phones that support is still very low and many times it is not a viable solution. However, we hope that this might change.
So - is native SRTP support next on the to do list? ;-)