WebKit's HTML5 <video> player is a nice one. Let's get caption/subtitle-track support in there.
Caption and subtitle support will be in the HTML5 spec fairly soon, see http://www.whatwg.org/specs/web-apps/current-work/multipage/video.html#timed-tracks a proposal.
Note also that we already have WebKit specific caption properties on HTMLMediaElement: webkitHasClosedCaptions and webkitClosedCaptionsVisible. These are implemented in Safari 5 on OS X and Windows.
I'll be using this as a master bug to keep track of the upcoming implementation of <track>.