Martin Fowler's Domain Specific
Languages book contains some Ruby
examples in the Internal DSLs
section. You can read the work in
progress on his web site or if
you've got a Safari account then it's
available as a rough cut.
There are also some slides here
from an Agile DSL Development in
Ruby presentation by Obie
Fernandez
Metaprogramming Ruby: Domain-Specific Languages for Programmers (PDF here) from an OSCON '05 session by Glenn Vanderburg
There is a relevant chapter One Lair and Twenty Ruby DSLs in the ThoughtWorks Anthology which is available in PDF as an example chapter.
