I've seen a few startups use iPhone mockups on their homepage. I think it's a great idea. However, I'm always surprised that they forget to show a FULL-BATTERY and 5-BARS of signal reception. Another one is making sure you have that black seem going around the screen. Finally, if you need high-resolution photoshop layers, check out this excellent link: http://www.teehanlax.com/blog/iphone-gui-psd-v4/