Lowest long-term interest rates in 300 years?

The Guardian Online reports on new research by Professor David Miles, Morgan Stanley economist, suggesting that "long-term interest rates [i.e., yields on 30-year government bonds], adjusted for inflation, could be at their lowest for the past 300 years." He isn't able to explain why, apparently, but the implication is that such low levels are not sustainable.

Charlotte Moore, "Low rates mean long-term liablities," The Guardian, March 14, 2005.