Some people noticed that since a few days, they seem to get logged out of de.wikipedia after leaving the site or restarting their browser, even though they checked the stay-logged-in box. Another symptom is that mostly the main page seemed to suffer from that, and they'd get logged in magically by viewing a new page. The problem is browser-independent. Only de.wikipedia seems to be affected.
If you can read German: http://de.wikipedia.org/wiki/Wikipedia:Fragen_zur_Wikipedia#Komisches_Auslog...
Sounded like a caching issue to me. I could reproduce the problem, restart Firefox and be logged out. Force-reload "healed" his, and I got the main page with my user links again.
Did someone change squid/caching settings around the 13/14th? Maybe on the European squids?
Magnus