Looking over the Debian Ruby Policy.  If you're interested, a good
jumping point is here:

  http://pkg-ruby-extras.alioth.debian.org/policy.html

According to the policy 3rd-party packages are installed directly into
/usr/lib/ruby/1.8/, right along side all the standard ruby files. I
realize that the Ruby distribution itself is split into multiple
packages for Debian, but it still seems like it would be better if
3rd-party packages where stored in their own space. When installing
packages manually they are placed in /usr/local/lib/site_ruby/1.8/, so
I would think a good place for 3rd-party Debian Ruby packages would be
in /usr/lib/site_ruby/1.8/.

In my particular usecase, being able to distinguish between what is
standard  to Ruby from 3rd-party is helpful because I know that the
standard libs are all tied to the Ruby version (i.e they are all
"1.8"), whereas 3rd party libraries will have their own specific
versions. If I can't distinguish between the two in the file system
then I am forced to pretend that each standard lib also has it's own
specific version.

T.