As a disclaimer, this is not a Xiph codec, but I was asked to post information about Ogg/Kate on this wiki. As such, please do not assume that Xiph has anything to do with this, much less responsibility.
Kate is a codec for encapsulating karaoke and text data in an Ogg container.
It has support for text (utf-8 only for now), motion (a set of curves, which can be Catmull-Rom splines or segments for now, to be used primarily for karaoke), and style/placement information for the text. Motion data can be mapped to that placement if needed (eg, relative to the font used).
The format is still in flux, so there is no code to download yet, but I have a proof of concept patch for MPlayer to use Kate streams multiplexed with Vorbis/Theora data as subtitles, though it seems to lose sync sometimes when switching languages (with a Kate stream per language).
I'm mostly creating this page to get used to that wiki.
More info will be posted soon, probably in less than 24 hours.