I really like that gmb can use various backends for playback.  Do you have plan to add support for xine http://www.xine-project.org/home?

not currently.
I took a look at the perl bindings (http://search.cpan.org/~stephen/Video-Xine-0.18/lib/Video/Xine.pm) a few months ago, but they currently lack too many features to be usable in gmb.

For some reasons, mpg123 uses a lot of cpu power on my system.  I use xine to play all my video files, so I hope you'll consider xine in the future

mplayer(-nogui) works pretty well for me (both movies and gmb), have you given that a try?

yes, I'd tried mplayer, it works well....it's just that I've gotten used to key bindings in xine.  ;)