Smuggling a low grade "homemade" or stolen nuke into a major city is one thing I can see within the capabilities of a terrorist group.
Nuking a Carrier Battle Group that is in a high stage of alert and on war footing is a whole nother ballgame.
Ask the Soviets. It would either take a very good attack by a nuclear attack class submarine (not to many of them in terrorist hands), a large and I mean large, modern, and extremely capable surface battle group (and I doubt that even the Soviets could muster that kind of naval surface firepower now), or a massed attack by Naval Air assets on the order of hundreds of simultaneous over the horizon launches by Backfire bombers (again something that barely theoretically existed in the numbers required during the cold war).
The offensive and defensive air power of a CV group, combined with the defensive capabilities of Aegis class Cruisers and Destroyers, make an EXTREMELY potent force.
To get a Carrier Battle Group you would either have to do it by suprise, a la the USS Cole attack, or be very very very lucky.
[ 09-11-2001: Message edited by: Vermillion ]