You are going to see a lot of sports devices, such as GPS and heart rate straps touting Bluetooth 4.0 or Bluetooth LE (low energy) hitting the market soon. Unfortunately, there is one big problem - Devices can only pair with one other single device.
Many athletes and coaches are used to ANT+ and it's ability to allow multiple sensors to cross-talk with each other. For example, a coach can use a device to monitor an athlete's cadence while another device is mounted on the athlete's handlebars for him to watch himself. With Bluetooth, that can't happen. The cadence sensor is "locked in" to the coach's device, so the athlete is out of luck.
This is the nature of Bluetooth itself as a security and anti-crosstalk feature. Once you pair your wireless headset to your phone and start talking, you don't want somebody else trying to pair as well and listen in. In comparison, ANT+ "broadcasts" it's data for anybody to pick up.
Admittedly, this is only needed by a percentage of athletes. But, this percentage is on the elite end of the spectrum and has a lot of "pull" with development. It will be interesting to see what comes along to solve this problem. In the meantime, be aware of the issue and choose wisely!