U.S. Subprime Expropriation?

I saw an interview in Businessweek of Martin Feldstein, where he expressed concern that a legislative modification of interest rates on mortgages underlying collateralized mortgage obligations and other mortgage securities, intended to protect vulnerable mortgage borrowers, might impair the creditworthiness of U.S. debt securities, and that set me thinking.  These securities probably meet the definition of "investment" under at least some U.S. BITs.  Would the impairment of the underlying debts constitute impairment of the "investment"?  If so, the next question is whether the legislative modification downward of the interest rates on these securities is an expropriation.  If it is, then the U.S. will be required to provide compensation to foreign investors of BIT parties.