I did not remember howto disable the boot video on my N900. I searched some time as most of the instructions were for using another video, not disabling, and i was not shure if just removing may end up in an unbootable device.
So to come to an end, i found how to disable the video here
You have to edit /etc/hildon-welcome.d/default.conf that it looks like this:
[hildon-welcome] # filename=Hands-v32-h264.avi
Like in most Linux config files the # is for commenting the line in Maemo, too.