better UI for nested working copies

There's a recurring question on #svn along the lines of:

% svn st
?       foo
% svn add foo
svn: warning: W150002: '/tmp/danielsh/wc/foo' is already under version control

I'm assuming the usual case is that foo is an independent working copy (question: could this be caused by someone deleting an svn:externals entry?).
And this seems to confuse people. so maybe there is a UI improvement we could do here.

There's some #svn-dev discussion here:
http://colabti.org/irclogger/irclogger_log/svn-dev?date=2012-12-06#l322