Initially my N900 was not reachable via its hostname in my LAN.
Later I installed "Pidgin Extra Protocols (Bonjour and Sametime)" from Extras - and I updated the firmware to PR1.1.1.
Which of both made it happen? I don't know, but I can connect to my N900 from my Macs using its hostname.
(The other way round still does not work: My N900 does not recognize my Macs by their hostnames.)