Mike - U.S. bonds aren't redeemed at the option of the holder. They are of varying maturities (bills, notes, and bonds), most of which pay interest every six months and then are redeemed, automatically, at maturity. So these bonds are not grants. The foreign governments who hold them are receiving interest payments and principal, and using that income to buy more bonds. However, I don't think the foreign holders of these bonds account for a very high percentage of the debt. Individuals, both foreign and American, as well as the Federal Reserve itself, buy more bonds than foreign governments (I'm pretty sure, at least).
Post WWII, the U.S. dollar became the world reserve currency. Other governments used the dollar as if it were gold, and only the dollar itself was redeemable for gold (and only in transactions between the U.S. and foreign governments or major financial institutions). In 1971, the convertability was severed, but foreign governments have continued to use the U.S. dollar as if it were still backed by gold -- as if it were gold itself. Old habits die hard.
There are probably some here who would disagree, but from my understanding, a strong dollar is actually good for these foreign governments. After all, a weak dollar lessens the value of their holdings, and is also unfavorable for them in international trade. I think the foreign governments, thus, want a strong dollar. They are propping it up by their continued purchase of our debt.