Krasnikov reckons...
...that Ford and Roman didn't actually compute the warp-field itself, but added up the vacuum energy around it - which is naturally enough going to be HUGE because it's huge anywhere.
That being said there are lots and lots of problems with the Alcubierre metric, involving space-time shockwaves and the like. But I saw a paper on the preprint archive that suggests a new approach that might dodge the old problems.
Adam