I htink this falls into that "great mysteries of life"
category. I'm sure there is a technical reason that perhaps one of
the Captivate deveopers could offer. But since they aren't usually
visiting these forums, I'll take a stab at it.
I think when you press Tab, the next logical element that Tab
would move to gets the highlight. In your case, it's the back
button in the Playback control. In my case, it's the progress
indicator. I'm guessing that you are either using a playback
control that doesn't have a progress indicator or you have turned
Unfortunately there is no way I'm aware of to influence the
behavior. Nor is there any way I'm aware of to change the highlight
color from bright yellow.