@-D-
While i'm not disagreeing that Silkcut is a brilliant demo, and deserved to win, the tunnel is a little bit of a cheat

To me, it looks like an animated texture mapped onto the inside of a cube. Normally, still textures are mapped onto the inside of cubes to give a panoramic view (often of a sky backdrop in demos). Here is an example: (requires quicktime plugin)
http://www.panoramas.dk/fullscreen/fullscreen32.htmlYou can see this if you look carefully, because the tunnel rotates more smoothly than you travel down it.
So, it not an animation per-se, but the tunnel itself is an animated texture.
Note that this is how it APPEARS to have been done, I haven't seen the source code or anything :-D