Given that a large part of investments in US treasuries and bonds are by foreign (especially far eastern) investors, what makes you think that a falling dollar (thus reducing the value of their investments) would encourage them to buy more.
There is a consensus out there that the massive US current account deficit will only be fixed by a significant depreciation of the dollar (thus reducing imports). This was emphasized by the fact that at the last G8 meeting, they spoke about an orderly fall in the dollar to reduce 'global imbalances', i.e. US consumers spending too much.
What I can't understand is how, in this environment, people are stilling buying dollar-denominated assets when it looks likely that, through currency devaluation alone, they are going to lose significant value in the next year or so.