I won’t bother rehashing all the rumors, but I would like to extend my previous thoughts about Apple and tablet computers.

The fact of the matter is that there does seem to be a significant demand for mobile computers. Many computer manufacturers have tried to meet this demand with “netbooks”: very small versions of laptops.

The trouble is that laptops aren’t really mobile computers; they’re portable desktop computers. If you want to use a laptop you put it down on a surface and lean over the keyboard and screen. It’s terrific that they work sitting on surfaces as lopsided and off-balance as your lap while lying on the couch, but the mechanics are still pretty much the same as the desktop computer.

A portable computer can be moved when it’s not in use; a mobile computer is designed for use while it’s moving. The iPhone interface isn’t good because it looks good and works well in the “white vacuum” of the commercials. It’s good because it works while you’re carrying a bag, listening to music, and strolling down the street. More substantial sessions with mobile computers are characterized by the extraordinarily low cost of a context switch: there’s no need to establish or clean up a work space. You can read a Kindle while waiting in line at the grocery store knowing you’re not going to hold anybody up.

The most surprising thing to me about the iPhone is not all the nifty little mobile applications—I would have guessed that it’s possible to do those well. What surprises me is how competent the iPhone is at things that I had previously considered to be desktop tasks: web browsing and email. It turns out that the touch interface is more than sufficient to navigate a web browser, and most web sites are perfectly tolerable on a smaller screen with the iPhone’s excellent pan and zoom features. The on-screen keyboard is nothing like as fast as a desktop keyboard, but it’s easily enough for writing a few paragraphs of prose for an email.

The iPhone is competent at web browsing and email, but it’s not good enough to be competitive with desktop machines. Mainly, the screen is just too small. As mobile computers go, the iPhone scores big on “mobile”, but it doesn’t quite feel like a “computer” in the sense that it could be a replacement for much of my desktop computer use.

There’s a demand for some kind of a mobile computer that can do web browsing, emails, feed reading, and possibly even e-book reading. The demand doesn’t include a need for legacy applications (as supported by the fact that casual users actually accept Linux instead of Windows on the current devices that try to fill the niche). Apple has the most expertise in the mobile computing market. A tablet would be unlikely to cannibalize sales of either the iPhone or any of the Apple laptops. It makes sense that Apple will release something in this space.

The outstanding question is when. If Apple wanted lots of third-party applications available for such a device, then they’d need to announce it at WWDC in a week’s time: developers will be setting their schedules based on what is announced there.

But Apple doesn’t necessarily need third-party apps at launch. I’ve seen analysts talk about how the App Store is the key to the iPhone’s success, but of course the iPhone was a hit when it had only Apple applications. They could easily ship a tablet computer with nothing but Safari, Mail, and an eBook reader developed in-house in conjunction with Amazon, and wait six months to allow any third-party apps. In fact, this approach would allow Apple to set the standard for how tablet interfaces should work.

There’s a third option: Apple could ship with only apps from a few hand-picked developers given early access to the SDK. Developers Apple trusts to build quality interfaces, and to provide useful feedback on the APIs. Developers who are better at building certain kinds of must-have applications than Apple is. Developers who can be trusted to keep their knowledge of the device top secret throughout the project until the day the product—and their app—launches.

Anybody know what Brent Simmons has been up to lately?