Rickard wrote:
>At the webpage is a picture of the first restaurant and it displays a
sign saying (with no apostrophes):


Thanx, Rickard -- that is just what I meant to make notice of. Indeed, I too checked out the sign this afternoon, and it says McDonald's. Which is exactely like "newsagent's" or "chemist's".

My previous statement was just that native speakers often 'forget' about the gentivie-s, when it's in a very common name or expression. That is very typical of streets -- if you go to London, you'll find St James / St James'/ St James's Street or Park or whatever. 
When there is final s and no apostrophe, it may mean it was intended in the form of an adjective -- which could be the case in McDonalds Hamburgers. 

Anyway -- it's just spelling, nothing to worry about. But clearly, most people -- even Nancy, as I seemed to understand from one of today's posts -- saw McDonald's as a whole (not that there's anything wrong with that!).

-- Sara