I think what you mean is the DACP protocol? There is an implementation (and detailed information about the protocol) on this page (for an Android app):

http://dacp.jsharkey.org/

Also, I've found a client implementation on Ruby, which should theoretically work on the N900 as command-line app already:

http://gitorious.com/ruby-dacp

(There might even be a Qt-based UI for this already in the "qtstereo" directory inside the Git repo, but I haven't checked it out. Just a pointer in case anyone wants to try implementing something like this - I personally don't use iTunes for music playback )