My ol HTC had a nice app that turned the phone into a TV remote...cannot find anything like that for the Droid...anyone?
I think james is correct because i believe (have read on random internet sources) that the IR sensor is an IR Proximity sensor, the sensor used to shut the screen off when close to your face and an application called Face of Ring which, when the phone is face down, can mute all sounds
In the good old days, IR was a way to transmit data. Many cell phones and laptops had IR for this purpose. Those days have been replaced by bluetooth, and this is now an antiquated feature since the only "real" use for it now would be for a tv remote app (and lets be honest, that is not a good reason to include it).
Also, those sensors/transmitters were fairly large, and would increase the size of the phone.