I think it probably be useful if you suddenly came up with a good idea while ridding on the bus, but other then that it might be kind of annoying with the small screen and keyboard on cell phones. I hate to type out an entire essay on a cell phone.
I probably just phone my voice mail or answering machine at home and just record my idea. If my phone has a voice recorder which some cell phones have these days, I'll record it to my cell.
But, it might look kind of strange if people hear me recording an idea for an essay or other assignment.