Hi Stephan,
Presuming the expand/collapse thing is done properly, it should be golden. You'll find a lot of sites use this approach when they have multiple pages of content, e.g. a product page with specifications, reviews, technical details, etc.
I do this on my travel website. A great way to test to see if the initially-collapsed content is being seen and indexed by Google is to take a block of text from the collapsed section and search for it in double-quotes.
Here's an example: search for "At the Bora Bora Pearl Beach Resort you can discover the sparkling magic of the lagoon". You'll find my site there at #3 (Visual Itineraries), along with the other 1000 websites who've also copied the resort's description straight from the resort's website (yeah, I really shouldn't do this). So much for Google's duplicate content detection when it comes to text chunks...BUT I DIGRESS. That content you see is on the More Info tab.
Now, on to what "done properly" means:
- each tab should be in a separate div
- assign all divs a class which has style="display:none;" EXCEPT the currently selected tab
- have onclick handlers for the tabs that set all of the divs' classes to the display:none class, and then set the newly selected tab's div class to one with display:block or display:inline
And not done properly would mean something like changing the text of a div with Javascript onclick()....because Google won't see that text in the Javascript. It's got to be in the HTML.
That's about it. Not so tricky, really. And works well both for usability (no roundtrip to the server, not even an Ajax fetch!) and for SEO (lotsa yummy content on a single page for Panda).