After MANY unsuccesful attempts with record-my-Desktop and other applications I recently found a GREAT software that lets me record the Second Life video stream.
It's called glc
http://nullkey.ath.cx/projects/glc
http://www.linux.com/feature/143775
It manages to grab a better framerate, because at first it only captures (a sort of 'dump') without encoding, so it uses less resources. It also provides a script to encode the captured video to MP4.
Here are my recent short video, made using Flycam mode and SpaceNavigator mouse:
Saijo City in Second Life - The Refinery
http://opensourceobscure.blip.tv/file/1408179/
http://www.youtube.com/watch?v=0gobucafqq4
Therese Carfagno @ ARENA in Second Life
http://opensourceobscure.blip.tv/file/1403824/
http://opensourceobscure.blip.tv/file/1403680/
Vulcano residents meeting
http://opensourceobscure.blip.tv/file/1387261/
http://opensourceobscure.blip.tv/file/1386904/
In the Wastelands
http://opensourceobscure.blip.tv/file/1365275/
The What sim
http://www.youtube.com/watch?v=_30Gffpo4tk