Jeremy Grantham's writings have been value investor and asset allocator must-reads for a long time. In his latest letter "This Time Seems Very, Very Different" he discusses the stock market's higher valuation and higher profitability since the 1990's.
"In conclusion, there are two important things to carry in your mind: First, the market now and in the past acts as if it believes the current higher levels of profitability are permanent; and second, a regular bear market of 15% to 20% can always occur for any one of many reasons. What I am interested in here is quite different: a more or less permanent move back to, or at least close to, the pre-1997 trends of profitability, interest rates, and pricing. And for that it seems likely that we will have a longer wait than any value manager would like (including me). "