If you’re designing anything that’ll be viewed on either the iPhone or iPad, you’re going to find LiveView a godsend. It’s been out a couple of years, but I only discovered it this week.
It comes in two parts - a Screencaster app for your Mac, and a companion app for the iPhone/iPad. As you work on the graphics, you can view it live on the device. An essential, and free tool. I shot a quick video on my iPhone to show how it works:
Quick demonstration of how LiveView works from Jon Hicks on Vimeo.
So far, it only seems to be lacking an option top use it in landscape mode.