The Books-on-line for SQL Server 2005 makes this statement regarding the limitations of XML data:
The stored representation of xml data type instances cannot exceed 2 GB in size.
It's correct, but it could be clearer. You could read this as "all of the instances combined can't exceed 2 GB in size." or "any given instance cannot exceed 2 GB in size." Per Michael Rys, and some of my own testing, you can definitely have more than 2 GB of XML type data in a single table, but no single instance can exceed that limit.
In this weeks edition of Alter View, Create Function I take a quick look at the marvel at is the application XCopy feature with SQL Server 2005 Express.