The preview of the April (!?!) 2006 issue of Linux Magazine has an article http://www.linux-magazine.com/issue/65/Super_Underground_Desktop.pdf saying that Reiser4 is the root filesystem for the distribution "Underground Desktop" http://www.ludos.org/ but says "it failed to live up to the expectations we had for Reiser4", blaming Reiser4 for some performance problems. Not much detail is given.
It also says that "Grub does not support Reiser4", so LILO is used. (Is that true?) Do you know of any other distributions that comes with Reiser4 by default root file system? Peter