One of the more usable softphones for Mac OS X is X-Lite (which also has Windows and Linux versions). Unfortunately something in the release version (3) didn't work with MacOS X 10.6 (Snow Leopard). But fortunately a couple of weeks ago they released a beta version (4.0) which will run on MacOS X 10.6. (The 4.0 Beta won't run on Mac OS 10.4 or earlier, and won't run on PowerPC Macs either, but presumably version 3 will remain available for those users for a while.)

Amongst the more useful features of the current Macbook Pro line is that the 13" version, which has only a single 3.5mm audio jack, will support both headphones and a microphone (ie, a full headset) through the same connector, using the same 4-part 3.5mm audio plug as the headset that comes with the iPhone (and hence any other adapter using the same setup). (In contrast the older Macbook that I had at my previous job didn't do this, and while it had both an input and an output audio jack, the input was at line levels rather than microphone levels.)

Which means that the combination of X-Lite and a SIP account and a phone headset (which I prefer using for phone calls these days) makes for quite a useful portable phone. And combined with OpenVPN the PBX can be colocated on my hosted server with good connectivity, and the phone used from wherever my laptop happens to be.

On a similar topic various people have reported that the iPhone TTY adapter (MA854G/A) is apparently the right wiring, etc, to work as a 3.5mm stereo headset (headphones+microphone) to 2.5mm mono headset (headphone+microphone). If so then in theory it may allow my Plantronics headset to be used on either or both of my Macbook and my iPhone, which would provide quite a nice headset. (The major problem with most softphones is that audio from speakers and built in microphone tends to be fairly poor due to the poor separation and hence echos/feedback, whereas using headphones and a good microphone provides near perfect separation eliminating the feedback.)

Also, to my surprise, JB Hi-Fi (in the bottom of the "BNZ" Centre in Wellington) has quite a few of the genuine Apple addons at rather budget prices (at least comparable to the Apple Online Store, unlike many other retail stores). It looks like a good location to get things like headsets, sync cables, and the like.

ETA: Unfortunately there are issues with calls dropping after 30 seconds in X-Lite 4 Beta 3. This appears to be due to not recognising incoming RTP (and/or a lack of RTCP from the provider) (I've seen this in the SIP BYE message to both Asterisk and Freeswitch). It appears that X-LIte version 3 had an option to turn this check off, but X-Lite version 4 (beta) does not. (Freeswitch doesn't seem to do RTCP at present, but is sending RTP packets back; Asterisk seems to have RTCP in versions since mid-2006.) (CounterPane's Bria product also has similar issues in some situations.; workaround, which unfortuantely doesn't seem accessible in X-Lite.)

X-Lite 4 Beta 2 (announced here) appears not to suffer from this problem, so that's what I'm using for now.