It’s a common frustration – you’ve designed a website only to check it out on an iPhone and realise the design looks all screwy. Rather than troubleshoot the CSS with the phone itself, it’s possible to use Apple’s own iPhone Simulator to test and re-test your designs.
Sadly, the iPhone simulator isn’t easy to find and is only (as far as I’m aware) available as part of the iPhone SDK. So, first of all; head over to http://developer.apple.com/technologies/xcode.html to download it.
Once installed, it’s still not exactly obvious where the standalone simulator is so here is where you need to go…
Macintosh HD/Developer/Platforms/iPhoneSimulator.platform/Developer/Applications/iPhone Simulator.app (Sept 2011: note on later versions, it’s now called iOS Simulator.app)
You can then launch Safari with mouse clicks and find out what your sites are going to look like on the iPhone. Then weep, when you realise you’ve got more work to do tweaking all the little CSS inconsistencies…
Some quick tips for usage: to rotate the iPhone left or right use ‘CMD + Left Arrow’ and ‘CMD + Right Arrow’, double-click with the mouse to simulate double tapping and you can mimic pinching the screen for zooming by holding down the ‘alt’ key (option key in the US) and dragging your mouse. 
