You can get little inserts to adapt a schraeder-drilled rim to accept a presta stem properly.
aluminum version:
http://www.bikemannetwork.com/biking/p/RM9002
rubber version:
http://www.bikemannetwork.com/biking/p/RM9001I
another rubber version:
http://www.loosescrews.com/index.cgi...id=40029521248