IntelliJ and the location of the Ruby SDK on Ubuntu

I've just built IntelliJ's new Ruby plugin (it's so new, you have to check it out of SVN and build it yourself).
I first got it running on Windows where on the 'Choose SDK' step of the wizard I selected Ruby SDK and pointed it at C:\Ruby which worked just fine. IntelliJ figured out it was Ruby 1.8.4 and all was happiness and light.
Then I tried the same thing on Linux (I'm using 6.06 Dapper). A whereis ruby listed /usr/bin/ruby and /usr/lib/ruby. Obviously, /usr/bin didn't work since that ruby was a file. Starngely, /usr/lib/ruby didn't work either.
After randomly trying to point IntelliJ at various sub-directories, I decided to compare the directory stucture with that of my Windows installation which had worked out of the box.
Turns out /usr was the directory which matched c:\ruby. When I pointed IntelliJ at /usr, it picked up the Ruby SDK right off and everyone lived happily ever after.