Pass XML around tiers

I'd love to see the responses to this question:

"This is a simple one - how do you pass XML between layers and/or tiers? I'm not looking for the philosophical answer. I'm looking for the physical answer. We can debate the philosophical later." [News from the Forest]

I typically avoid passing around XML but when I have in the past it's been "string" or "XmlNode'. I like to do the parsing up front and pass around structs or arrays. For large documents I just pass the URL or Filename.