According to this article, Georgia Tech researchers have found it possible to turn a nearby iPhone into a key logging device. Their process not only utilizes the accelerometer, but also the gyroscope that made its debut in the iPhone 4. The same researches stated that without information from the gyroscope, accuracy was much reduced. I find this to be a great example of someone thinking outside the box. Do we need to worry about this?
Personally, I doubt it is a huge risk. The researches themselves made the statement that it was difficult to achieve. I think another couple of points are more relevant that that statement though. The first being the statement on accuracy. The claim is up to 80% accuracy. Keep in mind that this is after feeding the output through a dictionary to clean it up. So if your users are using good password techniques (chuckle), the likelihood of this successfully producing a password is slim to none. However, it is possibly a way to get other interesting and/or confidential information that could be found in a dictionary. Also, it is worth mentioning that the dictionary could be customized as relevant.