Perty sure this is it: http://www.filmtools.com/butyl.html

Looking at the MDSS its listed as Architectural Glazing Tape.

The site itself describes it thusly:

Black Butyl Setting Tape is an all-weather preformed joint sealant made of Butyl rubber material in a flexible 5/16" square rope form. Butyl was originally used to seal window glass into autos, special effects technicians use it as a source for small flames. Butyl comes with a plastic coated protective release paper that removes easily. Reusable as an adhesive.

Yeah, I think thats the stuff. Now to find it locally.