While the details of emission assessment are useful, surely the key question 
is the potential for ships for modal shift (e.g. from planes to ships), and 
the question from that is where is this going to be realistic.

Ships currently compete with trains and planes across the Channel (emission 
figures anyone?) and do so because they take trucks. Longer ferry trips 
(e.g. Norway, Santander) also rely on carrying vehicles (and almost 
certainly produce very major savings in terms of truck and car miles 
substituted); foot / cycle passenger journeys are probably a relatively 
small part of the package. Train / ferry packages (e.g. via Harwich) offer 
competitive deals compared to places which high speed rail doesnt' yet reach 
etc. Freight by boat needs much more work (e.g. the new Edinburgh - Europe 
route)

But the key is of course long-haul and a focus on the Atlantic crossing 
makes sense. And here surely the issue is how far significant numbers of 
people will buy in to spending a week doing what now takes a day. People 
value time very / too? highly.   The long-term answer is a major cultural 
paradigm shift, but waiting for or engineering one of these may not be 
everyone's idea of a solution to climate change.....

One answer clearly is to make the journey an end in itself  (thus the QE2) - 
this is maybe where the 'burger & chips' option suggested by George doesn't 
deliver (lessons maybe here from the 'troop-ship' approach in WWII when the 
Queen Mary was fitted to carry several thousands on each trip).

In a wired up society being away from the office may not matter (a 
cyber-cafe in transit across the Atlantic....) but what would people in 
transit really want to do to make up for the perceived 'time loss'?. Other 
options that occur such as study weeks, retreats, minor surgery seem to be 
likely to be of limited interest only.

One more problem may be reliability. Storms in the Atlantic are likely to 
get worse... Planes fly above them.

I'd suggest on the basis of these issues that significant trans-atlantic 
modal shift to ships is not likely to happen or to contribute much in terms 
of the total number of passenger journeys made (Air ships may be another 
thing altogether).

This suggests in turn that the challenge here is not modal shift but 
reducing the desire to travel (I personally don't believe that top-down 
controls and rationing are lilkely to work without far far more public 
engagement on this issue than we have at present). High quality video 
conferencing may work for business but not for tourism. back to the paradigm 
shift?