Machine tags are simply tags in the form of namespace:predicate=value. They are currently supported on Flickr and Upcoming, and provide a convenient, readable approach to cross-domain and cross-context tagging that maintains some sense of an original context.
It has been proposed that machine tagging relationships would be a means to transport relationships for individuals between DiSo sites:
flickr:rel=friend flickr:rel=family twitter:rel=followed dopplr:rel=fellow-traveler xfn:rel=friend foaf:rel=knows
It's also possible that other uses for machine tags could emerge, for example for indicating user accounts on remote sites:
twitter:userName=factoryjoe flickr:userName=factoryjoe flickr:userId=25419820@N00