An announcement about the 0.92-alpha1 release of Konverter was made at
www.kde.org today.
Konverter is a front-end to mencoder, and is used to "convert almost every
videofile into the most common different videoformats and codecs. Beside
converting, Konverter also gives you the possibility to scale/crop your
videos, change codec details or just changes the audio format of your
file."
The changelog for this release says that it includes new features like
filesplitting, filemerging and preview.
(http://www.kde-apps.org/content/show.php?content=9884)
I haven't tried it, but something like that might be useful for theora
someday.
Regards,
John