How transportable are WebIDs ?

Can i ask a question here even though i'm not a member of the working
group?  This may be covered in the new spec, but can't find it.

My understanding is that WebID works because it ties "a User Agent to a URI
by proving that one has write access to the URI".   I can see how that works
where the write access to the URI is provided by a single service, like for
example foaf.me or openlinksw.com.  But how does it work where a different
service is asked to bind a different user agent to that *same* URI?   For
example can i get a URI and a certificate installed in my Firefox browser
from foaf.me,  and then on my  iPod bind that same URI to the Safari of my
pod?  But let's suppose that the foar.me service can't bind the Safari in my
iPod to that URI, but that openlinkssw.com can.   Isn't that not a valid way
that people will expect a transportable identity string to work for them?
Has this been demonstrated to work?

If this is not the proper place to ask this question, i apologize, and
request redirection.

Seth Russell
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