We've looked at Shiva plug for some specific applications like managing remote door entry and the like. The Ubuntu/Debian version of SAIL will run on Shiva we think although we haven't done it and it may still be a fair bit of work to cross-compile asterisk for the Shiva's CPU.
The Sark500, which is based upon PIKA technologies' Warp (PPC not ARM) is a better telephony vehicle but a fair bit more expensive than Shiva plug.
Kind Regards
S