I spent a little time yesterday reading about using cell phones in the classroom. I'll do a more thorough write-up of this later, but I think one of the most interesting potential applications is using cell phones as an iClicker replacement. Sure there is an extra layer of complexity there in getting SMS responses in quickly especially if you're in the basement of a building with low signal strength and the question of how to deal with the less than 1% of students who don't have cell phones, but at the same time, you have a pervasive technology that students already use on a daily basis.
To loosen it up from the more formal polling of just certain questions, what about live tweeting in a classroom? You can use twitter from your cell phone or from a laptop. There are many conferences that provide a hash tag for that conference so everyone can follow what everyone else at the conference is tweeting about. So how about the professor set up a hash tag for their class and run a live feed either just to their monitor to see what students are or aren't understanding, or if they're feeling really adventurous, run the feed to the projector so the whole class can see. You could minimize purposefully distracting tweets by having students register their twitter account beforehand, so if someone puts something dumb up on the screen, you'd know who it was.
Of course, the type of professor who would kick someone out of class for participating in a discussion would retire before allowing something like this in their classroom, but they're not really the market for something like this anyway. It needs to be professors who actually want feedback from their students.