Explicit "attribute=value" tags are more flexible than the W3 approach
to turn the entire document ID into a big long string. I guess it
depends on whether you believe you are dealing with a big database
or a big file system. Both approaches have their place. Again as
a simplified case you have "udi=//host:port/path" as a MIME identifier
and all is well.
I expect that MIME will be available in many e-mail products over the next
3-5 years. Since the only application that has anywhere near universal
appeal on the net is e-mail, it strikes me as only appropriate that
hypertext systems try to get as much leverage from mail as they possibly
can.
--Ed