I don't think ratings matter when da boyz can print it up. Risk here is a spike in real interest rates that would probably be accompanied by a strengthening in the US dollar (like 2008) or some foul play by the i-shares people. There is an interesting article about TLT on the Economic Policy Journal site that discusses some of the monkey business they are up to. Don't know how really important it is.
Can't link it because our boards are now IMPROVED :).