I've been playing around with Slow Shutter Cam, which seems to be the premier iPhone app for that purpose. It's not truly a slow shutter speed, but it simulates it pretty cleverly and allows some neat in-camera photos. This one is me spinning around in a chair. It took a bit of experimentation to get something interesting.
I find it neat that the EXIF data shows an exposure of 4 seconds, even though it's many exposures blended together, or however the app works exactly.