I trade stocks, it's a large part (majority?) of my income. The only metric I care about is the market value of my shares over the book value (ie. percent increase). It's the only thing that counts (since it directly reflects my profit).
Edit - milestones are nice, they're a bit of a psychological boost - but ultimately are worthless (except for said psychological boost they give to other traders, thus potentially driving up price).
I think it's a second-order, behavior-finance oriented line of thinking. I know that stock price is basically arbitrary and market cap is a much better measure of value. (And arguably, earnings and growth rate are even better than market cap.) However, I also know that many people consider $1000/share to be an important psychological milestone, and that it will command news headlines when the stock makes it. And so, understanding that the stock market is to some extent a beauty contest and other peoples' perception of a stock's momentum influences the stock price, it's rational for me to believe that when it hits $1000 it will go up, at least in the short term. It's also rational for me to feed into and cheerlead that perspective, because as a Google shareholder, I benefit when other people believe Google stock is going up.
The real headline should be: Google hits $330 Billion market cap, making it the 3rd most valuable company in the world.