Dude... yields plummeting is a bullish indicator. Less pressure on interest rates. Low yields equals less risk. Now if T-Bill yields went up 75 bp's, that would be a little worrying.
Sure there's some knee jerk flow into safety. But if the flows weren't going into T-bills then that would indicate people had totally lost confidence in the dollar, a far more bearish scenario aka 1987.
However the one big threat in the US econony is property valuations. Take 30% off the valuations of single family homes and you'd have half the country technically bankrupt. The rest of the world have gone through 50% declines before (eg. Japan and Australia late 80's, Hong Kong late 90's). I'm just not sure its going to happen to that extent, unless interest rates go spiralling up and the Fed loses the ability to supply the short term money by lowering the Fed Funds Rate.
The markets have been remarkably resilient, and a lot of bad news is already out.