In case you were unaware of it, the Pragmatic Programmers have a beta
book called “Enterprise Integration with Ruby”. Here’s the link:
http://www.pragmaticprogrammer.com/titles/fr_eir/index.html . It has
several sections on interacting with XML using Ruby code. It’s not
specifically Rails, but I imagine it’ll have what you need, all for the
price of a pdf ($20).
Also, the O’Reilly XML site (www.xml.com) has some articles on dealing
with XML in Ruby and/or Rails.
I’m sure others here will comment on REXML and the other associated
libraries, but the book goes through all that in detail.