I'll collect stuff you can do here - it doesn't necessarily mean that it's a good idea, though.
First off:
There's no good point in animated backgrounds - ok: maybe showing off ;-)
Major defects are
a considerable (and useless) CPU/GPU load
a distressing workspace
That said: there's a theoretical chance to have them with BE::Shell.
What you do is to use mplayer and use BE::Desk as output window, maybe sounds complex, but just try
mplayer -wid `qdbus org.kde.be.shell /Desktop winId` <some_video>.avi -loop 0
Notice that "-loop 0" must be last option, or you'll suffer from some flicker.
Maybe you also want to use "-fps 15" to slow down speed (and lower cpu usage) - whether this works out depends on the content of the video.
Ok, to get some geeks wet: you need to install the xscreensver package (you probably have)
The hacks are usually found in "/usr/lib/xscreensaver" - your distro should know.
There're many cool ones (all take --help), but we'll try the one called "glmatrix" =)
(There're more on The Matrix. What a pity they never made any sequel to this ONE* great movie ...)
/usr/lib/xscreensaver/glmatrix -window-id `qdbus org.kde.be.shell /Desktop winId` -fog -waves -rotate -delay 80000
"-delay 80000" will limit the updates to ~12fps (now as you're a geek you'll be able to do some math and figure why)
Now try "xmatrix" and "matrixview" yourself