How safe are TIPs, really

The premise behind holding TIPs is that they're the safest investment there is and I see both Larry's and Zvi's insistence on this in their writings both as correct and as a valuable service to their readers.

On the other hand, Larry routinely writes about the magnitude of the US fiscal problems. I thus wonder about whether one should plan for the contingency where the US government defaults on the TIPs.

Were this something one should contemplate, what are the implications? To invest in a basket of TIPs from several countries? To invest in real assets the value of which are highly correlated to the CPI? To assume this extremely unlikely and thus forget about it?

I found another way of asking this question in a forum inside assetbuilder.com:

What are the risks to TIPS if: (1) there is a massive run on the dollar; and/or (2) the dollar loses its status as the world's primary currency; and/or (3) the government ever defaults on its obligations?