Optimism couldn't sustain itself on the S&P 500 (SNPINDEX:^GSPC), as the broad index rose more than 1,592 points by early afternoon before falling to 1,585 by the close. Part of the fall can be blamed on Federal Reserve Chairman Ben Bernanke, who pointed out at a meeting of the Financial Stability Oversight Council that there were still vulnerabilities in the markets and that regulators need to remain vigilant.

Given the rather slipshod manner of financial regulation of late, investors should take it to mean that they should remain vigilant -- and really, that should be the case regardless of what Bernanke says.

Despite Bernanke's regulatory warnings, plenty of stocks took off today, many on good earnings reports. Let's look at three of today's best performers to understand why the market went all in, and whether it might last.

Infinera (NASDAQ:INFN) gained more than 23% after a decent earnings report, which showed stronger-than-expected earnings and a narrower-than-expected loss per share. Revenue guidance also looks solid, and these results encouraged analysts at Needham, who now rate the stock a buy with a $12 price target. That's a solid 46% above today's close, indicating a whole lot of opportunity left for investors interested in one of Fool analyst David Meier's favorite stocks.

Fusion-io (NYSE:FIO), which happens to be another of David Meier's favorite stocks, popped by 17%, also on solid earnings. The solid-state hard-drive maker trounced the estimates on top and bottom lines and appears very close to breaking even. Forward revenue guidance is ahead of the Street's expectations as well, and investors also appear to appreciate the company's recent purchase of a small privately held hybrid-storage company, which should help Fusion-io expand its reach in the highly competitive hard-drive market.

Cliffs Natural Resources (NYSE:CLF) gained some much-needed relief from a long, devastating decline when it posted better-than-expected earnings that beat very, very weak expectations. The mining company reported a steep year-over-year decline of 74% on the bottom line, but adjusted EPS of $0.60 still came in at nearly double Wall Street's expectations. However, the company continues to suffer from weak commodity demand and the associated weakness in pricing power, which executives are now expecting to dent revenue for the rest of the year -- although no one can say for sure how much of a decline it may be. Cost-cutting moves can only take a company so far.

Fool contributor Alex Planes holds no financial position in any company mentioned here. Add him on Google+ or follow him on Twitter, @TMFBiggles, for more insight into markets, history, and technology.

The Motley Fool recommends and owns shares of Infinera. Try any of our Foolish newsletter services free for 30 days. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.