This question has more than just academic or cocktail party relevance.
From Portfolio.com's Market Movers:
It's still too early to tell whether the stock-market crash of the past couple of months has done any lasting damage to the conventional wisdom that a buy-and-hold strategy applied to a diversified stock portfolio is generally a very good idea. But it is worth pointing out that a couple of the names one might associate with the idea, when you look a bit more closely, aren't nearly that simplistic.
For instance, consider Henry Blodget, who wrote The Complete Guide to Wall Street Self-Defense for Slate back in 2004 -- something full of common-sense advice for long-term investors. He recently posted a blog entry headlined "Time To Abandon Buy And Hold? No: Time To Buy", in which he seems to advocate at least some attempt at market timing:How does a normal retail investor know to sell stocks when they're expensive? Through rebalancing: once a year or so, look at your portfolio and, so long as you can do so without too much in the way of adverse tax consequences, bring it back to whatever kind of big-picture asset-allocation setup you wanted all along....MORE
The Japan news is profound and disturbing, but it does not change the basic equation: price matters. Buy stocks when they are cheap, sell them when they are expensive.