[xml] XML "diff" tools?

[greg_corson playstation sony com]

Can anyone recommend a good XML aware "diff" tool (libxml2 based if
possible) that can handle things like diffing two files where the tags
have been reordered (legally, within limits of the schema), whitespace
changed, attributes in tags reordered...etc?

The small amount of reorganization some XML tools do seems to drive most
diff tools crazy, making them nearly useless.

Thanks!
Greg