50 years ago would be 1959, but your point is correct, there is no real comparison of 2009 in the USA to anything. It is likely this downturn will be for gotten in a few years just like the last one in 2001-2002.
Take a look at the 100 year chart rallies have remained overbought for years, does that not mean an oversold condition can last for years also? Think about it. What on god green earth could cause a 50% rally?????