Shoe Pouch for Nike+ iPod Sport Kit, and Nike+ SportBand (2-Pack), Also Fits Adidas miCoach Speed Cell and Garmin Foot Pod
I was recommended this by a friend, and have already told other friends about it. I alternate running shoes, so I bought 2 - one for each pair.  I found that lacing it in upside-down (logo underneath) made it easier to pop the sensor in and out when I change running shoes.  The velcro is very sturdy, and the pouch 'hugs' the sensor...so I feel very secure that it won't fall out.