I found this application posted on Samuel Fisher's Facebook called Posture Man Pat. I believe this program could be potentially useful for composers or other losers like me who can't seem to extricate themselves from their laptop.
How does it work? According to the website, it's:
Very simple, it uses your webcam to track your face on the Y-axis of your screen. If your face drops a certain amount, then it triggers a warning.
After I downloaded and installed this program (it's for Mac only), this "warning" beeps at you and emits a flash. You can adjust these settings so that you either hear a beep or see a flash.
Also, this only triggers once if you have bad posture. What you need to do is straighten yourself up and hit the spacebar to reset the settings for it to work again.
Or, even better, I could stop using my laptop in bed.
[N.B. This software was created in Max/MSP/Jitter, which is not surprising once you see the layout. This also proves Max can do anything.]