It's often useful to know the signal strength of a Bluetooth connection. For instance, my Magic Trackpad becomes much more responsive when it's on the left-hand side of my MacBook Pro, rather than the right. It took me some time to figure this out, and it would have been a lot easier if I could have easily measured the signal strength myself. Until now OS X hasn't given any easy way to show the signal strength.
But 10.7 adds an easy display of the signal strength! Go into System Preferences » Bluetooth. The window show a list of all your connected Bluetooth devices, one per line. Click on a device and then press the Option key, and a bar graph and number will appear on the right showing the current strength. These are displayed and recalculated as long as you hold down the Option key. Like Wi-Fi signal strengths, the number is a negative integer, and numbers closer to zero are better. For instance, -50 indicates a strong connection, while -80 is very weak.
[crarko adds: I tested this, and it works as described. I also verified this is not available in 10.6.8.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=2011091514050082