tbone,
That I don't know. I am hooked up to a PC so I can't speak to how it might interface to an imac (i don't think this will be a problem given that it already is somewhat integrated to itouch/iphone). There is a category on the SONOS user interface labeled "music library" (both on SONOS controller and itouch). This is supposed to be the location on your home network where you store YOUR music. Under that category label is this (in order):
artist
albums
composers
genres
tracks
imported playlists
search
folders
Assuming that your computer is always on, you should have access to your library from the SONOS unit from anywhere within range of a zoneplayer (white box pictured above). The only thing I'm unsure of is mapping it directly to iTunes. You may have to copy your library of music to an external harddrive and direct (or point) SONOS to that harddrive on your network.
That said, the info you provided in regards to itunes is telling and I would proceed with caution given the rigid walls that apple tends to put in place. At the worst, you could begin to create new playlists or recreate your popular playlists directly on SONOS by use of Rhapsody. I can create and save a playlist of about 400-500 songs in about 10 minutes. Clearly, you are an avid music fan and this functionality would only add to your library.
Funny side note, now that friends/relatives know I can queue up anything at anytime, I've lost control of music in my own house. I've got some younger cousins that come over and my queue is suddenly filled with Britney Spears.