This actually happened two weeks ago, but I kept forgetting to send an email about it: XML importing has been added to the Data Transfer extension (now at version 0.2), so that now it allows both export/display and import of XML. And the extension's name is finally justified. :) Importing is done via the new "ImportXML" special page. The XML has to be in essentially the same format as the one Data Transfer uses to export data. This new functionality isn't meant for transferring content from one wiki to another, or for backing up one's wiki; MediaWiki's "Export" and "Import" pages already handle that. Instead, it's meant for transferring data from a non-wiki system, like a database, into a wiki. You would just need to write a script to print out that data in the correct XML format, and then the "ImportXML" page does the work of creating wiki pages from it.
You can read more about the new functionality, and see download instructions, here: