...
> Sure enough, it looks like apple didn't ship the tk extension:
...
> The obvious workaround is to build and install your own ruby
...

Thanks super, Ryan.  I was hoping to try to avoid that, but... :)
For those who are interested, the best instructions I found for
installing Ruby-Tk on Snow Leopard were at Dan Benjamin's Hivelogic
site:
http://hivelogic.com/articles/compiling-ruby-rubygems-and-rails-on-snow-leopard/
A virtual thank you to Dan :)

Craig