It would solve a problem for me if there was a port for the Mac.
Good news. The official last.fm client really is as bloated as people say.
For whoever was interested, I was keeping track of the songs I played on iTunes. Not anymore. The plugin kept asking me for a password I had already repeatedly supplied, and there was no real value in people knowing what music I'm into. It made me feel too self-conscious about my musical tastes ("quick", I would think, "press the 'next' button before people think I like that song" or "I'd rather people didn't know that song was on repeat for 3 hours").
That said, last.fm is a really great concept, especially the radio part where you could listen to music recommended based on either your or someone else's tastes. It's also a good way to more objectively show people what you're into. I tell people I'm into hip-hop, but based on almost 4000 songs played, hip-hop only makes it two times in the top ten (M.I.A., listed twice, counts as one, while DJ Shadow counts as the other). Turns out I'm more insterested in retro new wave dance electronica of the French, British, Australian and American varieties.
But really, I just want to listen along with what my friends from thousands of miles away are listening to. Along with them. It's not a technical limitation—the technology to do it is currently in place—but rather a legal (copyright) one.
Hopefully there will be a real API for it.
Looking forward to when Audioscrobbler comes out with a real API.
As soon as Audioscrobbler gets a real web service API, I'll use the PHP library on my site (this one is a web service, with a Python script).