I found a nifty explanation (written for scientists and engineers) of the proper use of the restrictive versus the non-restrictive clause. It uses (naturally) set theory.

The confusion between “that” and “which” is very common, even among professional writers and journalists, and many otherwise superlative bloggers (I’m looking at you, Mr. Den Beste…). Read it, and improve your writing almost instantly.

This really isn’t a nitpick, like dangling participles and split infinitives. The words really do mean different things, and wrong usage actually changes the meaning of the sentence, though it’s usually possible to figure out what the writer meant. The author of the link above has another page in which she explains why it’s so important.

Anyway, the use of “which” for “that” is always jarring to my eyes, particularly because I generally expect the former to be accompanied by a comma, and it never is when used incorrectly.