If you're a UI designer for iPhones you'll know how much time is wasted by saving your mockups and then putting them up on your iPhone to see how they look. It's not an ideal way to make sure your mockups look great.
Design Duet is a very simple and small app which solves the issue. If you're running Photoshop CS5 and above you'll be able to make use of the remote connection feature. Design Duet connects to Photoshop via remote connection and shows your current PSD on your iPhone. There is a second or two delay, but it's never been a problem for me because I first look at the computer screen to make sure it's ok and by the time I pick up my device to look at it it's there.
It's made designing mockups a hell of a lot easier and quicker - because you never know what it will actually look like when it's on the small iPhone screen.
Unfortunately the app only supports iPhone for now. I'm hoping it'll support iPad very soon and maybe even go over to Android where it could be even more useful.
At £1.49 the app is a steal, especially if you make money from creating mobile interfaces - it's a minuscule cost.