It's true that Google is able to crawl and index content that's contained within a hidden div. Obviously don't use this to hide a load of text on a page to manipulate search results - you'll get caught if it wouldn't pass a manual review - but it's useful to know this if you've got a legitimate reason to do so (e.g. you've got a navigation that isn't revealed until a link is clicked, or a box that displays the contents of the hidden dev when a "More Info" link is clicked, etc).