user@domain URNs, independent from OpenID spec

[meepbear *]

In the context of leaving comments, people would still want to continue 
using whatever method their using now which most likely includes listing an 
email address:

"PersonA (personA at domainA.com) wrote:"
and
"PersonB (personB at domainB.com) wrote:"

One is an OpenID, the other is an email but there is no way to tell the 
difference. You can show the OpenID icon next to it, but unless the person 
reading it happens to know what OpenID is and happens to know that OpenID 
IDs look the same as an email address they're going to think it's one.

I also don't really see how you could easily differentiate between the 
following using an email address format:

http://www.example.com/foo/bar/
http://www.example.com/foo.bar/
http://www.example.com/foo.bar
http://foo.bar.example.com/



Another case: someone owns http://www.domain.com and decides to host someone 
else under http://hostee.domain.com who wants to use that as their OpenID. 
Under the email scheme, there is no easy way this will work.

hostee at domain.com won't work because hostee.domain.com and domain.com are 
totally different, unrelated sites owned by different people.
@hostee.domain.com might work, but what would you put in front of it when 
someone wants to use the root path?

As a side note: regular expressions are not portable between programming 
languages so they can't be an option to provide mapping.