I've been using your gem to at work to handle tree data structures (which is great by the way - thanks for putting this together), but I needed to be able to merge trees that come from different sources (but share a common root identifier).
I've added this functionality in this patch - if you like it and think it's useful it would be great if you could include this in the gem. :)
… node. Resolves feature request #28688 (http://rubyforge.org/tracker/index.php?func=detail&aid=28688&group_id=1215&atid=4796).