I found a great post from Tom Keating about Asterisk Termination and ENUM, which discusses the [lack of] progress in the potentially lucrative market of Peer-to-Peer Enterprise Telephony.
From the article:

Of course, the Asterisk system uses its proprietary IAX protocol for inter-Asterisk communication and not standard SIP, so unless IAX is supported by all IP-PBXs (not […]