An easier way to do that would be to download the iPhone SDK from developer.apple.com and use the iPhone Simulator on a Mac while connecting the projector to your Mac. I did a presentation of a mobile website using that on my MacBook just a few weeks ago. Of course, it all depends on the equipment you have available to use.
The AV cable should work with all iPhone's, and the VGA cable works with the iPhone 4, but you can use the cables to display apps that support them. Most apps, including Safari, don't support video output, and you definitely can't show the home screen.