If you are developing/designing iPhone apps, the this may be something you will find useful. A company named Coohdoo makes this cool product that allows you to sketch on. Now I know there are templates for this that you can print out and use for free, but imagine how much paper you are wasting!
Anyways, check these Dry Erase, Magnetic templates at http://www.cohdoo.com/
The templates have faint gird lines, are 6.8" by 3.5" and have magnetic backing. A pack contains 10 Phone Doos.
They also have browser versions in the near future.