I’ve noticed this expression has a new meaning lately.  Urban Dictionary notes it’s currently slang for something so new and fresh it is like the latest fashion right off the store’s shelf or hanger.  The meaning has expanded to anything cool or exciting.

Wiki.answers.com says the older meaning comes from fishing slang; if you’re on the hook you are caught, trapped, or obligated.  Off the hook means you escaped.  Phrase Finder has several definitions, including some no longer in use.  But I found no origin for the phrase.