I'd like to keep it in the vpopmail project. The daemon could be part of the regular code and the php client module could be part of contrib?
This would cause problems. Then it would not be in PHP releases, and only in the contrib directory, thus making it still an "remote" option and not likely a widely adopted one.
I don't see much value in being part of the PHP release. You might convert a few people who need to pick a new mail server, but I suspect _most_ PHP developers aren't allowed to touch the mail server. A place in the vpopmail contrib is more likely to be found by the people who are deploying vopomail.
We need to make sure contrib is mentioned in INSTALL and/or README! I'm embarrassed to say how recently I found out about it...