I'm flying to the US next week [ex-LHR]. I've just checked our ESTA registrations - they're still valid until either the 2 year point or until our passports change. Print out the confirmation screen.
It's also worth logging on to ESTA and making sure your destination address is still correct ... you don't want "Men in Black" staking out a hotel you're not using this trip.

..................

Anyway, you still have to fill out the Green Card [I-94W] on most routes. Remember, this keeps Americans in employment, and thus saves the Free World from ... something.