It's free software — in both the beer, and speech senses.
You can use it anywhere in the world, subject to signing-up to a suitable local SIP-based voice-over-IP provider and being near a usable wifi access point. In many countries, you can find those for free or nearly-free — e.g. with SIPGate, in the UK, you can call to your PSP from any standard phone (landline or cellphone) for just the standard local call cost, and from PSP to another SIP phone (another PSP or a PC) for free.
It runs on both slim and fat PSPs.
It uses open standards, for maximum compatibility with other phone systems.
You can use a Go!Cam (Chotto Shot), Talkman microphone, or SOCOM-style headset for audio input. (And even if you don't have any of those, you can still send audio from a WAV file in a pinch).
It's still pretty much beta software — the audio quality needs some work (but it is usable), the UI isn't very pretty, and video calls aren't working (yet!). But it works, and it's a nice demo of what is possible".