I saw with much interest the new AGENT_API and NON-AGENT_API functionality.
It is a very important step in the right direction.
I have no doubt that we will talk about the project in term of its life before and after the API.
If that API could be extended to a degree where you could separate the development of the Vicidial engine from the web interface, that would be great for the project.
People that ask for more field or a different look could do it on their own.
Some other may feel more comfortable/interested in developing the web part in RubyOnRails or Flex, etc.
Some other may decide to stick to the PHP interface, but feel sure that they will not break the full functionality while changing some parts of the web interface.
On the other side, more time could be then put into improving and extending the engine.
I also think that it would make the API very powerful to be able to process and generate xml instead of the long list of parameters.
I do not know if you took the API step with that in mind, but nevertheless it has enormous potential. Congratulations!!
So, after completing the API development, or maybe during that process, the other frontier will still be to abstract the interactions with the underlying PBX so that Asterisk could be replaced by FreeSwitch or others.