From a central bamk standpoint, China & Japan are at the top of the international reserves list around $900M-$1T. Both have a huge trade surplus with the US. China, however, has bad loans totaling nearly $800M. Japan has been working on their bad loans since the bubble nicked in'89; not sure the amount, so I'll guess it's around $300M. They accumlate dollars thru trade, then sterilise by buying bonds. Thus, they have a vested interest in keeping rates steady to lower.
From a supply standpoint, the US 30yr. stopped issuing in '01, then started up again in '04. When the Nasdaq went kaboom, all these municipalities, pension/mutual funds and insurance companies immediately soiled their britches. Underfundings galore. Time for the safety and comfort of a long bond. England and France both introduced 50-yr. bonds to match pension liabilities, so don't be surprised if you see that in the U.S.
All this being said, the participation rates from CB's in the recent TIPS auction was 54%; for the Treasuries, its around 25%. I'd say the massive open interest is mainly due to the much-reported carry trade; borrowing in Japan and going long in the U.S. on interest rate differentials, for example. When markets gyrated back in May, these types of trades unwound a bit.