I've been working a Xanga archiving script. Thus far, in my testing, it can traverse all of a user's publicly-available posts (does Xanga even support private posts?), and download all of the comments as well. Each post has its own file, together with the associated comments.
The ultimate goal, I think, will be to help facilitate moving from Xanga to a superior blogging system, although I doubt I'll ever write code to automate that. Right now I'm just wanting to give users a way to archive their data, since Xanga currently makes you pay for that functionality.
At some point I'll make up a project page and everything, but I just wanted to see if anyone had any ideas or suggestions.
UPDATE - The project page is up.