Negative Nominal Interest Rates

We all know that real interest rates often fall into negative territory. In contrast, negative nominal interest rates are unusual…but then we do live in unusual times:

Yields sink close to zero at German, Dutch auctions, Reuters, 5 Dec 2011
Would Investors Pay for the Privilege of Holding Swiss Francs?, Wall Street Journal, 11 August
Bank Deposit Charge Sends T-bill Yields Back To Zero, Wall Street Journal, 5 August