Designing an iPhone app logo poses a big marketing challenge. The logo must be creative enough to stand out from the crowd in the App Store. Yet it has to clearly convey the function of the app on the iPhone's diminutive screen. It also has to tie into a brand's Web site and other marketing collateral. All this in a thumbnail's worth of space. Here's a look at some iPhone app logos that do the job well (and some that leave us wanting more than cute animals or screen clutter).