I'm a Project Manager, as such I often go to job sites to take measurements for whatever we need to build. This usual entails making a sketch, jotting down dimensions and such.
Imagine this: Holding on to a notepad, a pencil and a tape measure. Put the note pad down to take a measurment, put away the tape measure and picking up the note pad to note down the dimenssion, setting down the pad so as take another measurement... Hughh! Running out of room in the pad... Duhh!!! Hard to read due to too much info on one piece of paper. Arghhhh... you get the idea.
Now imagine this. This is how the app would work, it would be similar to AutoCAD. You touch the iPhone's screen and run you finger along it, you lift your finger and write down the desired length for the line, for example, 123.25". Boom, you have a 123 1/4" line. You get the idea.
iPhone in one hand, tape measure in the other, that is all you need. Moreover, you will end up with a correctly dimensional drawing that you can immediately e-mail back to the shop.
Don't forget to share the profits with me. This app will be a kiler app for the iPhone for many industries. Get busy.