Interesting thread on the XML-DEV list about Xml as a long term archival storage medium. Years ago I was trained as an archivist and so this topic was certainly of interest. The thread begins with a query as to what happens in 2027 with respect to Xml data. In this day, Xml is said to have disappeared from the scene (imagine the movie promo guy saying "...in a world where Xml has vanished...").
All that is needed to read and manage this Xml data is the ability to understand ascii or unicode. And in fact, that is the case.
I did some coding for the state of north carolina archives and records division prototyping and conversion of governor's records into Xml as a long term storage medium. So in archival terms (meaning 2127 instead of 2027) all that would be needed to read this material is a program that understands unicode.
Of course there is a physical issue too. One needs to be able to read the disk/harddrive/tape or other physical format in order to read the data as well.