St. Louis Fed 10-year Graph Since 1950

For all intensive purposes, the advance in the long bond price will have reached its limit with rates as low as they are now. With Operation Twist in place, this will ensure that rates remain low, regardless of currency fluctuations, I believe.

The unforseen changes will be in the short term rates, especially if they turn negative, as in negative nominal rates.

The long bond didn't trade before 1977 in the U.S. It can take up to 20 years after the crash for rates to finally begin turning up.

source: http://research.stlouisfed.org/fred2/graph/?id=GS10#