The current ActiveRecord::Errors#to_xml just spits out the full_messages. Though Rick has that working by pulling out bits of strings it doesn't really help non-ActiveResource XML consumers, and hey, it's XML, so why not put it in an attribute instead?
Below is my post to a discussion on the web standards group list I started just the other day.
(Awesome work BTW Rick. ActiveResource is coming together quite nicely!)