Data courtesy http://www.mbsquoteline.com
Invert that and you get an idea of interest rates being paid on those securities, spiking upwards. Hence, the spike in mortgage rates. This is another stomach punch to the housing market.
eta: The TNX (10-year Treasury index) is also doing some very bad things, jumping from 3.5 to 3.7 in a couple of hours, which is not quite as dramatically quick a move, but still a large move in a short period of time. This is again in a down stock market, which is not good.
eta2: Karl at Market Ticker has more charts, and some things to say about this, including an interesting one regarding quantitative easing.
eta3: The TYX (30-year Treasury index) is in on the game, too, up to 4.606%.