If this is ever developed inot a solution, just remember you heard it here first. Unfortunately someone has probably already suggested it and it won't work...but here's my $0.02 anyway :)
So I was reading the news about suggestions for stopping the BP leak in the gulf. Instantly I thought of an angioplasty baloon. Why not make a giant angioplasty baloon, insert it into the well and then inflate it?
Here's what I was thinking...take a couple hundred feed of steel tubing (say 1/3 the diameter of the open hole in the blowout) and perforate the lower portion and seal the "bottom." In the perforations put collapsed round balloons made of something like Kevlar (which has a strength of 3600 MPa or about 500,000 PSI). Inflate the baloons to seal the bore.
By having a series of small balloons (rather rthan a few long verticle ones) we would prevent the failure of any one balloon from causing the seal to fail. A smaller diameter pipe inside the external pipe carries something like concrete/bentonite (drillers mud), etc. to inflate the baloons and seal the well. Comething that hardens would be idea. Once the well is blocked, both the interior shaft and the void space (if there is any) between the exterior shaft and interior shaft could be filled with concrete as well.
Assuming you can pum the "filler" at sufficient pressure to inflate the balloons, it should be relatively trivial to get this kind of contraption in place (just shove it in the bore) and put it to use. Should be cheap to build as well.
There. I came up with an idea. Tell the Shrimp boat captains I tried. God help them folks, 'casue I don't think anyone else can.