On the chart below you can see that using the slope of the 2004 SPY trendlines anchored on the February 2010 market bottom gave a 2010 bottom trendline that nicely predicted the bottom (or at least a pause) in the recent correction.
The big question now is the top trendline. Of course there is nothing to say that 2004 will in anyway predict 2010, but there are macro level similarities (sideways period after a rapid run-up after a grueling bear market, general business recovery) as well as micro level similarities (2004 and 2010 SPY prices closed less than $2 away from each other today).
On the dissimilarities ledger, volatility is running considerably higher in 2010, Eurozone troubles could plunge that area into even more severe economic difficulties–dragging the rest of the world down, and there is some evidence that ETFs are disrupting the historical tendency of some asset classes (e.g., commodities, bonds) to behave differently than the equities market.
Given the scare of the recent correction (assuming it is over), I find it hard to believe that the market will quickly rally back into 121 territory for SPY. On the other hand, the 2004 top trend line is only a few points away at 114–just a couple of good up days away. I’m guessing the ceiling will be around 116 / 118 for SPY through this cycle.

SPY 2004 vs 2010, click to enlarge