[cobbzilla]

This information may be a bit dated, but I visited Berlin circa 2008 and my friend (American expat) who had been living there for 10+ years insisted that we not pay at the subway, because no one checked. Following his guidance, I never paid for the subway, and was never fined. This was just within the city of Berlin, though.

[tormeh]

This varies in place and time. I think close to 50% of my travels on a recent trip were checked.