If he means treasury bond prices are unsustainably high, sure... no doubt about it. Buying bonds that yield almost 0% is not a great investment.
But not every over-priced asset is a bubble. People are bidding up bonds out of fear to preserve capital, not in hopes of stupendous price appreciation. Since the yield on long-term treasuries will not go below zero there is only so much room for price appreciation and expectation of price appreciation is at the heart of any self-respecting bubble. (With bonds the higher the price the lower the interest yield)
Once the yield gets to zero the bond is just a much less convenient form of US currency. And a ten dollar bill will never be worth eleven dollars.
Since the goal is capital preservation rather than price appreciation I don't see any need for the "B" word. (Some Jews escaping Germany paid way over-market amounts of currency for diamonds they could sew ito their clothing to escape with. But it wasn't a diamond bubble.)
(Granted, you can leverage T-bills just like anything else and get into trouble. With enough leverage any bet can become a high-risk/high-return proposition.)
Buffet's observation about the 2008 Treasury 'bubble' does, however, add to something about the B-I-G bubble we have experienced. For the last 12 years a gigantic price hump has been running through all sorts of things in the direction of perceived asset-safety, running downhill from pets.com to cash:
1997-
Crazy speculative stocks> blue-chip stocks & real estate> consumable commodities: food, oil. etc.> US treasuries & Gold> Cash -2009
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NEW YORK (Reuters) - Warren Buffett, whose Berkshire Hathaway Inc (BRKa.N) (BRKb.N) sits on $25.54 billion (17.8 billion pounds) of cash, said worried investors are making a costly mistake by buying up U.S. Treasuries that yield almost nothing.
In his widely read annual letter to Berkshire shareholders, the man many consider the world's most revered investor said investors are engulfed by a "paralyzing fear" stemming from the credit crisis and falling housing and stock prices. Treasury prices have benefited as investors flocked to the perceived safety of the "triple-A" rated debt.
But Buffett said that with the U.S. Federal Reserve and Treasury Department going "all in" to jump-start an economy shrinking at the fastest pace since 1982, "once-unthinkable dosages" of stimulus will likely spur an "onslaught" of inflation, an enemy of fixed-income investors.
"The investment world has gone from underpricing risk to overpricing it," Buffett wrote. "Cash is earning close to nothing and will surely find its purchasing power eroded over time."
"When the financial history of this decade is written, it will surely speak of the Internet bubble of the late 1990s and the housing bubble of the early 2000s," he went on. "But the U.S. Treasury bond bubble of late 2008 may be regarded as almost equally extraordinary."
http://uk.reuters.com/article/wtMostRead/idUKTRE51R1Q720090228