Saving memory in with fork() and copy on write

I wrote a blog entry, as well as a tool, on saving memory in Ruby on
Rails using fork() and copy-on-write in modern Unix systems. I think
this may be useful to some people. This is the URL:
http://izumi.plan99.net/blog/?p=19

I've search quite a lot for similar solutions, but I seem to be the
first one to do this. It was fairly trivial, I only spent half a day
on this. I wonder why this hasn't been done before.