I'm working on this, but it's not going to happen untill first quarter 2006.
I have the same idea, first of all I'd like to use a wifi phone to create a setup that will allow wifi phones to be used with the wireless on pfsense and then either have a proxy or asterisk with a ISDN/PRI interface in the pfsense box. Then you have a pbx for the wireless phones, once this is done you could also use the asterisk as a PBX for wired ip-phones or softphones. My plan is to implement asterisk +a modified version of Asterisk Flash Operator Panel. And I also want it to use RADIUS for authentication.