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Posted yesterday at Marketwatch.
Summary: based on current market's P/E - both trailing and forward, Hulbert suggests that the market is either fairly valued or undervalued.
Of course, it's a simplistic analysis, but it's worth considering.
It may not be useful to try market timing on a broader basis, but if you don't have your asset allocation where you think it ought to be, perhaps an article like this can be persuasive in helping one feel comfortable rebalancing.
Now if anyone out there is interested in convincing me to be more comfortable with Treasury bonds, I'd love to hear the arguments for them...