Ok, I just did this myself. I had some trouble at first, but then I  
got it down pat. My instructions:

1. Download the source from Ruby-lang.
2. Uncompress it by using that mouse thingy
3. Open up Terminal (Applications -> Utilities -> Terminal)
4. type 'cd Desktop/#{your uncompressed ruby folder}/'
5. type './configure'
6. type 'make'
7. type 'sudo make install' and then type in your password

Now here's the fun part
8. type 'cd'
9. type 'sudo pico csh.cshrc' (if you're using bash, type .bashrc  
instead) and type your password
10. Using your arrow keys, move your cursor to the  line that says:
set path = ( $path )
and replace it with
set path = ( /usr/local/bin/:${path} )

press Control-X
press 'y'
press enter
11. log out, and then log back in
12. type 'ruby -v' and voila!