I only can speculate about the answer how the Droid Bionic actually implements this "in-pocket detection". Most Androids that have a light sensor, (which sometimes acts also simple proximity sensor) use this as sign that you either doing a call without a headset or that you have put your device into the pocket.
It is also imaginable that Android uses the information from the 3-axis accelerometer and/or the 3-axis gyroscope to detect in-pocket situations. But I don't think that this is actually done, because these sensors use quite a amount of power when in use.
For an example datasheet of an Android Light/Proxmity Sensor, see the SFH 7743 from the Motorola Droid. It detects IR-light around 900nm. It seems that some Androids have separated light and proximity sensors, while others abuse the light-sensor as simple proximity sensor (e.g. Samsung Galaxy S).