A super-slim laptop that uses chip-based flash memory in place of a spinning hard drive, of course. If the rumors are right, Apple (AAPL) will unveil one at the annual Macworld confab next month.

Before you begin salivating from gadget lust however, be forewarned. The rumors should be taken with a grain of salt (or a whole tub of it if you have one handy) — and not just because Apple prognosticators have predicted for years that an ultra-light dream machine is right around the corner.

The real reason to doubt is this: If Steve Jobs unveils a flash-based laptop in January, it could be his gutsiest move since the iPod nano. Why would a FlashBook be such a gamble? Because while it’s a cool idea, it’s not clear whether enough customers would pay the premium Apple would inevitably charge for such cutting-edge technology.

Yes, flash storage helps a laptop to do cool tricks like slim down, boot up faster and extend battery life. But there are tradeoffs too. For instance, flash handles data differently than a hard drives does, so software workarounds are needed for heavy-duty tasks like video editing. Marketing VP Greg Joswiak mentioned the storage performance issue to me during an Apple event this past summer, which suggests that the company is weighing the pros and cons.

Samsung’s flash storage drives can replace hard drives in laptops. Image: Samsung

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There’s also price. Consider that the MacBook Pro, Apple’s professional laptop, already starts at $2,000 with a 120-gigabyte hard drive. How much of a premium would a flash drive add? For a clue, take a look at Sony’s Vaio TZ Series, which includes models with 48GB and 32GB flash drives. Models with the flash drive option can easily cost $900 to $1,300 more, with a third of the storage space. As Apple seeks to expand its market share, it has to be careful about adding features that put its wares out of reach to all but the fattest wallets.

But don’t take this to mean Apple won’t put flash in its laptops at all. The company is notorious for getting components at bargain prices, thanks to the negotiating chops of CEO Jobs and the logistics prowess of chief operating officer Tim Cook. Because of the iPod and iPhone, Apple is already a very important customer of Samsung, the world’s largest provider of flash memory — so the company is in as good a position as anyone to get the best deal if it decides to add flash to the mix.

And Jobs is clearly not afraid to take chances; don’t forget the bold move Apple made with the iPod nano. Even though the hard drive-based iPod mini was at the height of its popularity, Apple replaced it with the flash-based iPod nano and a new design — a risk that boosted the gadget’s popularity even further.

If you can pay for the parts, flash-based laptops can deliver some sweet surprises. I recently spent some time with a laptop running Microsoft (MSFT) Windows XP with an Intel (INTC) Core 2 Duo processor, which Samsung sent me to demonstrate the performance of its 2.5-inch NAND flash storage drive. Though the laptop itself didn’t have a slim design to take advantage of the flash, I did notice that it went about its business silently, even as I launched and shut down programs. As a longtime laptop user, I’ve gotten used to hearing the pesky whir of a hard drive when I switch tasks, so the peace and quiet was a weird thrill.

Those kinds of benefits have led some high-tech observers to think Apple will add flash storage to its Mac recipe sooner than later. Richard Doherty, director of research at the Envisioneering Group consulting firm, thinks Apple is likely developing a flash-based touch-screen laptop reminiscent of the iPhone, which people in the graphics and publishing industries could use to share ideas. “The multi-touch technology is too good for just a 3.5-inch phone,” Doherty says. “If it’s not at Macworld, I think it’s something in the cards for the next year.”

Doherty also pointed out that because Apple controls both its hardware and its operating system, it could probably squeeze better performance out of a flash-based Mac than competitors would get out of Windows laptops. And it could be slimmer and more durable; a flash-based laptop would run significantly cooler than a machine with a hard drive, so engineers would not have to worry as much about adding design elements to help dissipate heat. If Apple doesn’t want to build a laptop with an all-flash hard drive, the company could take a gradual approach as well, and employ a new breed of “hybrid drives” that use a combination of flash storage and a traditional hard drive.

Whatever the solution, Steve Jobs will need to summon heavy doses of both engineering and marketing magic to turn an inevitably pricy flash-based laptop into a must-have item. Still, those who dream of seeing one on the Macworld stage next month can take heart; for Apple fans, January is when dreams sometimes come true.