Whoa! It's an exciting time right now with some white hot iphone apps becoming available.
This is my fave so far - http://blog.last.fm/2008/07/13/lastfm-for-iphone-and-ipod-touch - despite some reports of initial bugs.
Downside - not available in AU right now. I'm sure this will change.
What I would love to see is one of the local carriers getting behind an app like this, because this app (as well as the pandora iphone app) and the facebook app are already showing us the great potential the iphone has to truly blur the lines of media and devices. A portable device makes last.fm all the more powerful - this isn't just another iteration of the website, it brings the brand into a new dimension and it provides more value to the user.
According to Nielsen Netview, Last.fm is currently at 191k users in AU, with YOY growth of 138%. I will chase site side data from the last.fm guys.