A few weeks ago, Adam Lasnik posted in the Google Groups that the Google Webmaster Central team is seeking to answer popular frequently asked questions. The criteria: the questions should not deal with specific sites, they should be of interest to a wide range of webmasters, and they are not already addressed in a recent blog post or in the help center. There's a great number of questions so far, and Wysz has taken the initiative to answer one of the questions all about hidden text.
As I've noticed other users with similar questions in this group, such as "What if my navigation menu uses display:none to hide text until a button is rolled over?"
First, that-which-should-be obvious:
The reason we perceive hidden text as a problem is that it serves content to search engines which users don't see, and presenting different content to search engines and users makes it more difficult to properly rank pages. If we detect that this text is intended to deceive search engines, there could be a negative effect on how your site is indexed and ranked in Google, including removal from our index.
So what about CSS with display:none for navigational purposes? Wysz goes on further to say that it's okay as long as it adds value to the user:
Many sites use similar methods for navigational menus and other functional elements, so please rest assured that the existence of display:none on your site is not on its own a one-way ticket out of Google's index.
Rest assured that if you have the best intentions of improving the user experience, you're safe and sound with any hidden text.
Eric Enge has also written up a very helpful guide on hidden text at SEOmoz.
Forum discussion continues at Google Groups.