generally speaking when slime (smank) attempts to open a socket on port 0 what it means is "one port on any socket, i don't care which." on cmucl on osx (ext:create-inet-listener 0) does work.
[however there is another bug in swank:create-socket not returning the socket unless the MP feature was defined. since i don't know where helmut is taking the multithreading support i'm not going to commit a fix of this, but a temporary fix is to jsut comment the #+MP read conditional in swank::create-socket]
-- Marco Ring the bells that still can ring. Forget the perfect offering. There is a crack in everything. That's how the light gets in. -Leonard Cohen