Moving retirement funds - Bond Index or Treasury Securities?

With a significant sum that has done quite well distributed between an S&P 500 Index and a Dow Jones U.S. Completion Total Stock Market Index, I moved all of it to one that matches the Barclays Capital U.S. Aggregate Bond Index (which invests in government, corporate, and mortgage backed bonds) in early May.

Though it turned out to be a more favorable move than letting it sit there in funds whose value dropped, I'm starting to wonder if the chatter about another round of economic problems warrants yet another move to a more secure government securities fund. The borrowed "heads I win, tails I don't lose much" philosophy has been guiding my tinkering of my retirement account for the past 4 years. But I'm no expert on the vulnerabilities of this current fund to any imminent changes in the economic climate.

The history of this bond fund is that it has never lost money on an annual basis in the last 10 years. Its worst performance generated a 2.4% return in 2005. But the current economic climate doesn't seem to match up with events over the past 10 years, so I'm reluctant to presume another round of positive returns based solely on historical performance. Perhaps it has had positive returns during bad economic times for a good reason, but my limited understanding of that reason doesn't give me much confidence.

So what says the Money Board? Has an aggregate bond index gotten less safe? If you had only only two choices with your own money, would you rather have it invested in non-marketable US Treasury securities? Or would you keep it in the bond index fund? With my "heads I win, tails I don't lose much" philosophy, I'm not about to put it back in a stock fund right now. If I'm too risk averse, feel free to give me a reality check.