I was playing the campaign and noticed an extra yellow chevron when I was moving with the platoon (mission 2). I finally figured out that it's a personal position waypoint that keeps me (the player) in proper formation with the AI. I'd never noticed it in earlier ArmAs. Is it new?

Always been there mate.. its a position indicator, and not to be followed to the letter.. lol!
Sometimes its obvious that if you stick to that little chevron it can lead you straight into a killing area.
You will get it if you are a subordinate in a mission, but not if you are the leader..

I think it also depends on your difficulty settings; but it's been there since OFP AFAIK.

Yep, been there since the original OFP, although it was easier to see in that game. I wonder of the colour of the chevron in ArmaII/OA can be changed?