It works!

For those in a similar situation (where compiling Ruby on gentoo with 
the doc USE flag set causes ri to cease working usably), here's how I 
managed to get it to work. (Thanks to _ibz on #ruby-lang for his help in 
getting this resolved.)

1) Delete the /usr/share/ri/1.8/system directory and all of its contents.

2) cd to a Ruby source directory. (tar xzf 
/usr/portage/distfiles/ruby-1.8.2-preview2.tar.gz; cd ruby-1.8.2)

3) run "find . -name '*.rb' -o '*.c' | xargs rdoc18 --all --ri --op 
/usr/share/ri/1.8/system"

4) go get something to eat...this will take awhile...

Once that finishes, the ri documentation should be working correctly 
again. I'm still not completely sure why the ebuild isn't building the 
documentation correctly...maybe someday I'll have more time to delve 
into it.

Hope this helps someone.

- Jamis

-- 
Jamis Buck
jgb3 / email.byu.edu
http://www.jamisbuck.org/jamis