The interbank lending rate on Swiss Francs is now negative
The less realistic it is for you take delivery of a currency, the lower negative nominal interest rates can go.
Suppose I am an investor in Spain, how I am actually going to get my hands on, let alone store 10 Million in paper Swiss Francs?
As a note to more recent readers: the Yes Virginia, is a running theme from my old blog at blogspot years ago. During the very beginning of the financial crises there was a debate over whether or not interest rates could ever be negative, with some serious Wall Street types as well as academics scoffing at the idea.
My side was vindicated in late 2008 when the yield on US Treasuries briefly turned negative.