The new iPhone SDK comes with an iPhone simulator named Aspen Simulator, which is a full iPhone emulator and includes a few built in Apps including the iPhone Safari (Mobile Safari, Safari Touch?). Using this simulator is a sweet way to test how webpages look on an iPhone without using a scrunched up Safari window or an actual iPhone.

This appears to require a webserver though, as I haven’t figured out a way yet to just specify a path to an html file. Maybe it’s in the manual.