Does anyone know of something that will let me design or at least sketch out iPhone interfaces on the go? I'm at school about 6 hours of the day, and that's when I get the best ideas, and need to actually make a rough draft of my design.
You didn't say what level of technology you typically have available to you.
If you have an iPad, I highly recommend Penultimate. There are other sketch-type apps around, but this one is my favorite. Simple, and beautiful. There aren't any interface mockup templates or anything, it's just a freehand sketch app.
If you don't have an iPad, might I suggest going low-tech? I never go anywhere without pencil and notepad. My personal weapons of choice are the Moleskine Large Reporter, Plain, a Tul 0.7 Mechanical Pencil (which clips nicely onto the Moleskine's strap closure), and some iPhone templates (which can be printed out, cut to size, and tucked into the Moleskine). You could also throw in a stencil kit, but I usually just freehand it.