Bob, here's one of the best articles I've seen on the Texas crutch (that's what we're talking about):
http://amazingribs.com/tips_and_technique/texas_crutch.html
Like the author, I never foil ribs or pulled pork, because I've found it really doesn't shorten it very much, and the bark is far superior to foiling (just my opinion). Brisket definitely gets the crutch, though.