iPhone browser simulator 22 June
MarketCircle have released a Mac-based iPhone browser simulator. It's really just a mini version of Safari, but it's quite neat to use.
However, because it's not doing the fancy zooming that the iPhone's Safari browser will do, it's not a very exact approximation of how a user would interact with your site on their handset. The makers are promising more features soon, though, so it's worth keeping an eye on.
UPDATE: iPhoney 1.1 now includes a basic zoomed-out view, and has been released as open source.