We've been using the Content-Duration header with http servers to provide the length (in time) of a particular Ogg physical bitstream. This speeds up ui display and, if Content-Length is also set, allows faster seeking dispatch over remote protocols by priming the binary search with an initial average bitrate.
Older applications used X-Content-Duration instead. This SHOULD be supported by user agents for backwards compatibility with older server implementations, but new servers should use Content-Duration. This page is a draft spec for registering that message header.