I've seen the theory that you could work out which pages are loaded from wikipedia over SSL by looking at other metrics like content length etc, but thanks to stuff like gzip compression, caching headers etc, this is much harder to exploit in practice. Plus there's the huge overhead of maintaining a database to link the frequently changing metrics back to the appropriate page on wikipedia. There's a great link somewhere (which of course I now can't find) where somebody prototyped this idea and found it really pretty hard to implement.

In the event this was even tried, it would presumably be trivial to defeat with injection of random content somewhere in the server responses anyway. This of course all assumes we can trust the root certificate authority though :P