At the University of Southern California, fees are the
responsibility of the student.  Each organization (school,
institute) has one or two accounts to which tuition for GRA's and
TA's is charged.  There is no restriction on the number of units
for which a student may be enrolled (although I believe there is
a minimum imposed by the Grad School).  I was once in on a
discussion about this, and the concensus was that it would be
anti-intellectual to impose any limits (in terms of number or
field of study).

Elaine Steward