Is this implementation part of a wireless deployment? The only reason I can think this would be necessary is for a WLC to assign vlan information on the fly. If users are attached directly to the LAN it would seem more likely you would just assign their vlan to that port. You could make every port a trunk port, so when they authenticate, the switchport will know how to talk with their vlan but that still would not work (since pcs don't understand vlan tags) and you would open your network to major security issues.

If it is part of a wireless deployment, check this out:

http://www.cisco.com/en/US/tech/tk722/tk809/technologies_configuration_example09186a008076317c.shtml

I know it does not deal specifically with the SGE model, but if this is your setup then the switching infrastructure is the least of your worries.

Bill