Sorry to bother you mate but I was scanning the Sonos group and noticed that you are Aussie based with a Sonos and were having some trouble getting Scrobbling setup. I am based in Melbourne and has similar troubles. I had to contact the local distributor, Playback who enabled things for me (http://www.playback.com.au/). Send an email to email@example.com to ask for support in setting up the music services.
After they have set things up for you using your last.fm account details then you will need to use the desktop client to setup the service.
BTW - I subscribe to last.fm and it works perfectly through the Sonos here in Australia. It's worth it to find new tracks that you might not have heard from especially with the strength of the aussie dollar. I use last.fm as a music discovery service.
Hope this helps you out.