Canonicalizing relative URLs seen in URL type properties?

What are the rules for canonicalizing URLs seen in URL valued properties?  I
would guess that whether or not a value appears in an HTML URL attribute or
is encoded some other way with microdata, the rules should be the same.
 Which implies paying attention to <base> tags etc.  Is this correct?

--
John Panzer / Google
jpanzer@google.com / abstractioneer.org <http://www.abstractioneer.org/> /
@jpanzer