I found this graph interesting. One of the bullish arguments is that the earnings yields make stocks very undervalued especially when compared to bonds. A couple of things 1) That assumes bonds are priced correctly, a scary assumption and 2) That the earnings will be similar over the next 12 and 24 months as they are today - even a scarier assumption.
This graph shows the real earnings yield. I do not think it is all that informative in showing times to buy but it does show that the real earnings yield is not all that pretty.