A tip shroud rotating can be handled
quite nicely with current marerials.

Take a look at the new (Dec 2004) Flying Magazine.  Inside is an article
concerning the Skyship 600 - a Swiss blimp used during the Olympics for
aerial security.  There are 4 or 5 pictures of the shrouded motors.  They
look like the shroud is fixed while the fan blade tips are either attached
to a rotating hoop which fits or rides in a groove in the shroud, or the
tips are recessed into the shroud.
By fixing the shroud, rather than having it rotate, there are a number of
benefits that I see.
Reduced skin friction.
Gyroscopic forces kept to a minimum (reducing undesirable maneuverable
reactions and hub bearing loads).
The shroud and engine can be positioned to provide an adjustable thrust
vector.
Sorry, guys.  I just don't see the benefits of a rotating shroud for any
mobile application.
Kevin in Denver

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