Research In Motion's (Nasdaq: RIMM) PlayBook isn't the only tablet that will miss the 2010 holiday shopping season.

Tech blog Engadget reported over the weekend that Hewlett-Packard's (NYSE: HPQ) Slate 500 -- the $799 tablet that it's marketing directly to enterprise customers -- won't ship on new orders for another six weeks.

Did you think that the Slate was going to be a hot seller? I sure didn't, especially since it's not cheaper than Apple's (Nasdaq: AAPL) iPad and it packs a slightly smaller screen. Sure, there are also some notable advantages for the Slate. It's easier to connect to other devices, it has a pair of cameras, and it runs Windows and can stream Flash videos.

However, there's always something suspicious when a new product that doesn't seem to be hot is suddenly out of stock because of claims of "overwhelming demand."

Well, a tipster tells Engadget that HP only set out to make 5,000 of the gadgets. It was actually floored when it received orders for 9,000 units.

In other words, Slate isn't hot in a world where Apple moves a million iPads in less than a month. HP's Slate 500 may have been more than just a numerical moniker. It was almost an initial production run!

This is going to be an important lesson to keep in mind as Research In Motion's PlayBook and other tablets built around Google's (Nasdaq: GOOG) Android begin hitting the market. Unless the company is willing to give an actual fat number -- the way Apple eventually does -- don't believe the hype.

Don't fall for companies that blame shortages on demand, instead of their own production and fulfillment shortcomings. Don't buy into superlatives along the lines of "surpassed our internal expectations" unless those actual projections are being spelled out.

As a satisfied iPad owner, I'm not turning my back on the competition. There is certainly room for a spunky competitor to come in and either win consumers over on price or on specs. Dell (Nasdaq: DELL) and HP aren't going to let tablets cannibalize their entry-level laptop sales without a fight. One would be wrong to dismiss RIM's chances with its PlayBook early next year, given its tens of millions of BlackBerry owners.

A serious competitor will emerge in all of this. I bet it will be the first one willing to take Apple's route and actually spill the beans on the actual number of tablets that it's selling.

Do you own a tablet? Which one are you looking forward to the most? Share your thoughts in the comment box below.

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Longtime Fool contributor Rick Munarriz pecked away this article on an HP computer, but he does not own shares in any of the companies in this story. He 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.