One the most interesting features of smart phones is the accelerometer inside. Rather than attempt to explain how it works(see Wiki), I'll focus on what it could mean for the iphone as a design toy/tool. A graphic designer can now throw in gravity as a possible tool within even simple digital design environments. Add to that the touch interface, and the fidelity of the rendering, and that the rendering is immediate and happens in real time. Go try and do this type of manipulation in Photoshop so quickly and easily.
One interesting App is Sketchmania, where you can draw simple objects, lines, stick figures, etc. that you then can move around the screen using gravity as you rotate the device around. This uses the accelerometer, but in a very basic 2D way. Nothing to admire on the visuals front, but a hint at what could be possible within a better drawing program. Try the free version with ads, it's worth a doodle or two.
With Marbling, the 3D wonder of the accelerometer comes into its glory. This is more widely known for creating funhouse effects on snapshots of one's friends and loved ones.
These were BuroDestruct dwgs that were filtered in Marbling. Surprisingly smooth rendering, even without the resolution for now.
Next up are a few iphone shots that were also run through Marbling.
How about drawing with heat in real time? Heat Pad handles that. I just started to play with this, it's probably more toy than tool for designing but with a lot of features and color schemes available, and again with really nice rendering.
One more mention is Pollock, which is an 2D ink splat doodle App. Nice and organic, although very bitmapped for now.