Wall Street sure knows how to put on a halftime show.

Reviewing some of the best-performing stocks during the first six months of the year can be a real head-turner. Could you have ever imagined restaurants, telcos, coffee beaneries, and even an auto giant among 2009's biggest stars?

Well, going over the list of hundreds of companies that more than doubled during the first half of the year, we can see that a lot of the star performers appear vulnerable. The stocks have come up, but now it's time for the comeuppance.

I'm going to review four of the high-flying stocks that I feel are gliding too close to the sun. You may not agree with me, but that's what the comment box down below is for.

Palm (NASDAQ:PALM): Up 440% through June 30
I'm seeing a lot of full-page Palm Pre ads these days. Targeting Apple's (NASDAQ:AAPL) iPhone owners, the ads point out that many of their contracts are up, just in time to switch to the sleek new Pre smartphones. The ads also note the pricing disparities between the competing wireless plans.

There's a flaw in the strategy, of course. The iPhone owners who are wrapping up their two-year contracts are the early adopters. These are the Apple fans who paid as much as $599 for their phones. If they want, they can now upgrade to a speedier 3G version of the phone with the same 8 gigabytes of capacity for just $99, or go all out for a video-recording 3GS with greater storage capacity for as little as $199. Many have also invested in App Store programs, so they're financially vested in the iPhone platform.

It's going to be hard for any smartphone -- even the envelope-pushing Pre -- to woo them away. Perhaps that's why Apple cleared a million new iPhones in its debut weekend, while research firm JRPG figures that Palm moved just 84,000 units. The same firm expects 1 million to 1.2 million Pre smartphones to be sold this year. That's a great number, until you consider that the tally is less than the number of iPhones or BlackBerrys sold in a single month lately. This is a huge point. There are a finite number of people who can afford the costly data plans that make smartphones tick, and the Pre figures to be little more than a niche player.

Cynics will rightfully point out that I turned bearish on Palm in my "Throw This Stock Away" column nearly two months ago, when the spunky portable-computing pioneer was trading at just $11.17. It has risen by 33% since then. But I still stand by my bleak thesis. Once the buyout rumors fade away and the reality sets in that the Pre isn't conquering the world, Palm will surrender some -- though clearly not all -- of its first-half gains.

Diedrich Coffee (NASDAQ:DDRX): Up 6,506%
I didn't mistype. Diedrich really started out the year as a $0.36-per-share penny stock, and it closed at $23.78 six months later. The recipe to this jaw-dropping 66-bagger isn't necessarily homegrown. Diedrich is one of the many K-Cup coffee suppliers for Green Mountain Coffee Roasters' (NASDAQ:GMCR) Keurig single-cup brewers. In its latest quarter, sales to Green Mountain soared by a whopping 96%, accounting for the company's growth.

Keurig is hot. Green Mountain has sold nearly 1.2 million brewers -- not to mention hundreds of millions of K-Cup portion packs -- during the first six months of its fiscal year. It's great that Diedrich is one of the many java-sourcing beneficiaries, but even Green Mountain's stock has risen by a more mortal 129% through the first six months of 2009.

Diedrich has become more than just a piggyback play: It has become a pig. The K-Cup-fueled turnaround is real, but what will happen as more coffee brands hop on the K-Cup bandwagon? Green Mountain is the royal play here. Don't believe the coattail hype.

Ford (NYSE:F): Up 165%
Among the three domestic automakers, Ford is often portrayed as the healthiest. It hasn't been as much of a panhandler as General Motors and Chrysler have. It has a good chance of skirting bankruptcy -- something that obviously can't be said of the other two American automakers.

And there's the rub, my little hubcap. When GM emerges from bankruptcy, it's going to be in better shape to compete against foreign and domestic automakers.

Along the way, the auto market is still sputtering. Consumers are still weary of big-ticket purchases, and even idled factories and shuttered showrooms aren't eradicating the supply glut of new cars. Did I mention that the improving quality of cars is also lengthening the life expectancy of new rides? Yes, I said that with a straight face.

Builders FirstSource (NASDAQ:BLDR): Up 172%.  
If you think banking on new-car demand is a poor bet, imagine corralling all of your chips around demand for new homes. Builders FirstSource is a supplier of structural building products to the residential real estate industry.

You may have seen glimmers of hope on the housing front lately. Developers are posting narrowing losses. Order cancellations are down. Home prices are also starting to stabilize. None of these things ultimately matters, though. There is still a glut of existing homes. Speculators and condo-flippers who treated real estate like a Monopoly board game are clearing out, and it may take years -- yes, years -- before there's a buoyant new housing market at construction prices worth exploiting.

Analysts see revenue falling by 30% at Builders FirstSource this year. The few analysts following the company don't see profitability until 2011 at the earliest. If you really want some skin in this game, consider home-improvement superstore chain Lowe's (NYSE:LOW). It's at the mercy of many of the same catalysts, but at least it's consistently profitable.

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Longtime Fool contributor Rick Munarriz always sticks around for the halftime show. You never know when a wardrobe malfunction will happen. He owns no shares in any of the stocks in this story and is also part of the Rule Breakers newsletter research team, seeking out tomorrow's ultimate growth stocks a day early. The Fool has a disclosure policy.