The annual data are as follows for this period (Jul1975-Jun2011):
It appears there's a reasonable story one could tell about returns from AAA to BB: higher returns, and higher intuitive measures of risk: beta, volatility. But for B and C rated stocks, the returns make no sense to standard asset pricing theory, because these are obviously risky stocks. I remember presenting this chart to an NBER conference around 2000, and the esteemed audience told me I was wrong; my data had to be incorrect. I was working at Moody's, so my ratings data was as good as it got. Anyway, I wrote it up and sent it to Journal of Portfolio Management, and the editor, Peter Bernstein, wrote back they weren't accepting submissions at that time. I thought that was an odd response. This avenue wasn't part of my day job, so I let it go, but I keep updating my data for fun.
The result is really corroborated by Campbell, Hilscher and Szilagyi (2005), who found distress risk to be negatively correlated with stock returns, which makes sense because volatility and leverage is inversely correlated with future returns, and cash-flow is positively correlated with future returns, so those are the main drivers of default risk.
Reality is that which, when you don't believe it, doesn't go away, so I don't really mind when people tell me I'm wrong on facts like this.