Hi Mike, also see here for a script that will grow the selection from one selected face to its neighboring attached ones:
http://moi3d.com/forum/index.php?webtag=MOI&msg=6988.4

Set that on a keyboard shortcut and then keep triggering it until the selection has spread all the way through.

You can use that if you want to extract out the outer or inner piece of a hollow solid with a cavity in it.

- Michael