I can't encode my video files or music with Opus because you can't arbitrarily seek the track without backing up around 80ms and playing back up to the point where I want. So it isn't available in webm containers yet for that reason.
Perhaps you instead meant MKV? (now responding to the comment on frames/MP3) Yes. That is an issue for MKV with Opus. The problem is that the MKV container has no mechanism to signal a stream level or seeking level preroll other than just having frame boundaries. With Opus you need to seek several frames back and decode forward (not just a single frame) in order to converge. This isn't unique to Opus: Various video formats can be encoded with rolling intra (e.g. H.264) a mode which is useful for conferencing because it avoids fat keyframes. Since the movie 'backup' scene doesn't encode this way the mkv ecosystem's solution to these sorts of streams appears to be, so far, to fail to seek in them. I'm sure it will get worked out eventually.
The preceding comment is false: http://src.chromium.org/viewvc/chrome?view=rev&revision=...