Forgive me if this was already discussed here (its very possible I could
have missed it). But if not, I found this very interesting:

http://blogs.codehaus.org/people/rinkrank/archives/000994_annotations_in_rub
y.html

A most interesting quote from this article is:

  It took the Java community forever to do stuff
  like this. First a couple of years of XDoclet,
  then JSR175. And tons of code too. In Ruby it's
  1 hour's work and 25 lines of code. Go figure.

Curt