Of bonds, yields, stocks and PE

Quite pathetic right? I'm not sure if I can find any bond yield longer than 20 years old, because the website where I got the data didn't have such an option for me to choose. It seems like the 20 year bond gives us a yield of 3.40% - not exactly impressive, though it's the safest form of investment. The highest the bond yield ever went up to was 5.19%, back in 1990, for 5 year bond rate.
3.4% means a PE of 29
5.2% means a PE of 19
Since current 20 yr bond yield gives us a PE of 29, does it mean that once PE of STI reaches around 29, that pretty much forms the ceiling of the upsurge, since the inflow of money should rationally go over to bonds?
Not sure :)
Stumble it!