Certain periodic events mark the passing of time. The sun rises and sets. The weather goes from warm to cold to warm again. Football changes to basketball, then baseball. And, a couple of times a year, we see a new rumor that Apple will be coming out with a tablet computer. Because, you know, that Newton was such a huge hit that Apple's just sweaty-palmed to do it again.This time, ZDNet is spreading the tablet love, in a report that crept onto the Internet the Saturday of Memorial Day weekend. ZDNet writes:

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A little birdy tells me that Apple will announce a 12- or 13-inch tablet in the fall of this year. Most likely in the September or October time frame. It will run the full Mac OS X and have a slot loading SuperDrive, an "iPhone-type" GPS chip, and an Intel Core Duo processor, presumably Intel's Atom.

My only criticism of the ZDNet article is that its headline is too specific. They should have gone with a more general headline, like "Apple To Announce Mac Tablet," because a headline like that is one you can use and reuse dozens of times, every time rumors about a Mac tablet surface. ZDNet notes that there were Mac tablet rumors in 2004. Mac.Blorge reports on more of the history of Mac tablet rumors, noting that Apple has filed several patents.

Rumors of a Mac tablet dominated the Apple blogs during the run-up to Macworld Expo in January. I was pretty skeptical -- I wrote a blog post in November titled "Nobody Wants Tablet PCs:"

Maybe Apple is coming out with a tablet PC, but it seems unlikely. It's not a product that makes sense. Tablet computers have been available for Windows a couple of years now; they're not selling like gangbusters and there's no reason to assume that a Mac version will do better, no matter how much magic pixie dust Steve Jobs shovels onto the product.

However, there's something I wasn't thinking through back when I wrote skeptically about the value of tablet PCs: I like to surf the Web on the iPhone. The Mobile Safari browser makes the overwhelming majority of Web sites quite readable on the little iPhone screen, and the iPhone and Google Reader Mobile are a killer combination. I spend hours at a time, several times a week, surfing the Web on the iPhone -- and often I do it sitting on my couch, even though I have a perfectly good full-fledged laptop sitting on the coffee table. The laptop is heavy, it takes a while to start up, and the iPhone is more comfortable and easier to use if all I'm doing is reading articles on Web sites.

So wouldn't the iPhone be even better for Web browsing if the screen were larger?

Another thing that's changed since January is the emergence of the Asus EEE and other inexpensive sub-notebooks as a competitor to Windows-based laptops.

I still think that fully-functional laptop-replacement tablets are, at best, a niche market. Even the CEO of Axiotron, the company that makes the Modbook, the only authorized Mac tablet, agrees:

Apple is reportedly developing its own tablet computer, but Axiotron isn't afraid of the competition, [Axiotron CEO Andreas] Haas said. The Apple tablet will basically be a bigger version of the iPhone or iPod Touch -- a consumer device with a 7-inch to 10-inch display, running a cut-down version of Mac OS X with little or no third-party application support, with an interface that's lacking in fine control, in part because it will be controlled using the user's finger. "Finger painting is fun -- when you're four," Haas said....

Apple isn't expected to come out with a competitive device for the Modbook, probably because the market is too small. Gartner estimated that just 1.4% of the portable PCs sold in 2006 were tablets, and that figure is likely to be similar today. That's bigger than the market for the Newton when Apple shut it down in 1998, said Haas, who was the product manager for the Newton at that time. On the other hand, the market size for tablets is good for a company like Axiotron.

A full-fledged notebook-replacing tablet like the Axiotron model weighs as much as laptops, generates as much heat, and costs as much: $1,400 and up. The Axiotron Modbook is priced starting at $2,290. A nice Mac tablet would be much smaller and lighter than a notebook, use only solid-state parts, and cost, say, $800 or less. I'd buy something like that, and I think many other people would, too.