IR Proximity Sensor
Walking the /sys/class/ tree, there's something that identifies itself as an hsdl9100
root@castle:/sys/class# cat input/input3/name hsdl9100_proximity
That's probably an Agilent HSDL-9100 Miniature Surface-Mount Proximity Sensor. Palm specs say the proximity sensor is used to turning off the screen when the phone is put up to your face as you are making a call.