Ok, I have asterisk 1.4.17 running on the pfsense platform. It is happily gateing 50 SIP phones with no codec translation (gsm) onto an IAX trunk to our main switch. It is also a captive portal for 20 Internet Cafe machines and a handful of wifi routers. This is all running on an Acer Celeron 2.4Ghz with 1GB RAM. Load average with traffic has yet to go over 0.1 . I haven't done any exhaustive tests, but I have no reason to believe that my actions have crippled pfsense in any way.
This is not easy to do. You must have a fully installed 6.2-RELEASE machine available on the net to pull files from. pfsense has been severely stripped! Some bizarre things were removed (split? comm? those two probably saved about 10k of space

. You must be able to read the compile output and fix things as you go along, by pulling missing files from your fully loaded box. I wish I had made a list of everything I pulled over. Almost all in /usr/bin.
The hurdles were as follows:
1) used sysinstall to get at online packages, and installed the latest gcc and gmake
2) mv /usr/bin/cc and /usr/bin/cpp out of the way, then symlink to /usr/local/bin/gcc and /usr/local/bin/gpp respectively
3) download the latest ports tree and unpack
4) cvsup the latest asterisk source into the ports tree (was 1.4.17 for me yesterday)
5) make install!
As I said above, you will have to be able to read the errors in the build output and pull missing files. Thats the biggest issue. But it IS possible, and I am quite happy with the result so far. Granted I am not using any zaptel interfaces and it is pretty much exclusively for outbound calls, so I am not stressing asterisk much, or pfsense for that matter.
Want to hear more craziness? I got X running on this beast too. Long live FreeBSD.
Cheers,
j