"On 23 July 1992 Giotto operations were officially terminated after completion of final orbit adjustments and configuration of the spacecraft for its third hibernation. Only 1 to 7 kg of fuel is left on board, insufficient for any extensive future maneuvers. Giotto flew by the Earth on 1 July 1999 at a closest approach of about 219,000 km at approximately 02:40 UT (10:40 p.m. EDT, 30 June). The spacecraft was moving at about 3.5 km/sec relative to Earth."

Lovely google :)

iirc - the said that DI has >150kg of fuel onboard. An extended mission is certainly feasable - just a matter of how much money it would cost.

iirc - Stardust will be terminated after a correction manouver to avoid following it's re-entry capsule into the Earths atmosphere, as it is too low of fuel for an extended mission, and Genesis didnt get a follow up extension either
http://genesismission.jpl.nasa.gov/mission/solo.html

Doug