At The Motley Fool, we poke plenty of fun at Wall Street analysts and their endless cycle of upgrades, downgrades, and "initiating coverage at neutral." While the pinstripe-and-wingtip crowd is entitled to its opinions, we've got some pretty sharp stock-pickers down here on Main Street, too. (And we're not always impressed with how Wall Street does its job.)

Given this, perhaps we shouldn't be giving virtual ink to "news" of analyst upgrades and downgrades. And we wouldn't -- if that were all we were doing. Fortunately, in "This Just In," we don't simply tell you what the analysts said. We also show you whether they know what they're talking about.

Today, we're going to take a look at three high-profile ratings moves on Wall Street, which on Thursday went negative (to varying degrees) on each of Southern Copper (NYSE: SCCO), Suntech Power (NYSE: STP), and Corning (NYSE: GLW). Let's dive right in.

Copper loses its shine
First up -- StreetInsider.com is reporting that Dahlman Rose has just downgraded Southern Copper to "sell." Citing a 20% spike in the company's share price year-to-date, and pointing out that this is nearly twice the gains in copper pricing, and four times the gains on the S&P 500, Dahlman "Rose" up in objection.

Basically, Dahlman's making an "irrational exuberance" argument here. For one thing, Southern-C's gains seem outsized relative to copper's rise in value. For another, Dahlman points out that "a large portion of [Southern Copper's] growth projects are based in Peru, which has shown increasing signs of political and social instability related to mining operations." And for a third, Dahlman argues that at 7.5 times 2013 EBITDA estimates, Southern-C is getting a valuation more than twice what's accorded to fellow copper producer Freeport-McMoRan (NYSE: FCX), "a premium that we do not feel is justified."

I agree. And you don't even have to examine esoteric concepts such as "EV/EBITDA" to see why Dahlman is right about this one. From a plain-vanilla P/E perspective, Southern Copper's 13.5 P/E looks out of whack next to Freeport's sub-10x valuation. The more so when you notice that even the earnings Southern Copper does produce are of a lower quality than Freeport's, which backs up roughly 90% of its reported net income with cold, hard, free cash flow. Southern Copper's free cash flow-to-earnings  ratio, in contrast, is just 73%. I don't know that this necessarily makes Southern-C a "sell"... but I definitely know which stock I'd buy first. (And I have.)

Suntech sets in the east
Out next high-profile downgrade comes courtesy of ace stockpicker Stifel Nicolaus, a CAPS "All-Star" analyst who hung a "sell" rating on Chinese solar power play Suntech Power yesterday morning. The reason hasn't been made public yet, but I think we can guess that the latest round of subsidy cuts in Germany had something to do with it. Also, we learned that an organization billing itself as the "Coalition for American Solar Manufacturing" is accusing Suntech – along with Trina Solar (NYSE: TSL) -- of "dumping" solar panels on the U.S. market -- a charge that could result in tariffs and other penalties being imposed on the companies if a related court case finds against them.

So far, investors are shrugging off the downgrade and ignoring Stifel's advice to sell Suntech stock -- which was actually up over 10% percent over Thursday and today. I'm not sure investors should be so quick to discount Stifel's concerns, however. According to our CAPS supercomputer, Stifel is literally one of "The Best" stockpickers on Wall Street, you see. It's also probably right about Suntech, which despite reporting slight "GAAP" profitability, has a history of burning cash -- and is in debt to the tune of nearly $1.9 billion, or three times its own market cap ...

Corning shattered?
Last but not least, we come to Corning, which yesterday reported pretty miserable earnings to investors, and found its stock sold off to the tune of 10% in response. This morning, the news prompted a downgrade from investment megabanker Morgan Stanley, which now rates Corning "underweight."

This feels like an overreaction to me, however. For one thing, Corning had already warned us that this year was going to be a rough one, with improvements only likely to begin appearing in fiscal 2012-2013. For another, with the shares priced at just six times earnings today -- and even cheaper if you subtract Corning's massive cash stash -- it's hard to see how things could get much worse for Corning. On the other hand, with analysts only expecting 4% long-term growth at the company, any exceeding of these diminished expectations could cause the stock to bounce back quite nicely.

Foolish final thought
I'm on record arguing that Corning will outperform the S&P 500 over the next five years. Indeed, of these three stocks being downgraded on Wall Street today, it's the only one I'm confident will prove the skeptics wrong, and deliver outsized returns to investors. I've said Wall Street was wrong to sell Corning before -- and I stand by that position today.

And I'm holding myself accountable for this advice. You can track my advice on Corning and 55 other active recommendations on Motley Fool CAPS right here.

Corning had a disappointing quarter, but there are plenty of other companies out there that investors need to watch during this earnings season. In the Fool's "Fourth-Quarter Earnings Report: 7 Stocks You'll Want to Watch," you'll find information on this quarter's possible big performers. It's completely free for our readers, so click here to access yours today.