For an Apple Tablet to be a hit, it will have to be more than a big-screen iPhone. And the difference between a lithe, touch-based Mac and a giant, lame iPhone comes down to one crucial nerd-factor: memory management.
Memory management is boring to talk about. It's also boring to do. You're probably half-asleep just reading this sentence, but that's sort of the point; developers hate memory management. When they build iPhone apps, they have to control the iPhone's memory: what goes in, what gets stored, what comes out. When your program closes, your app is supposed to give back all that memory to the OS, so that it your computer can use it for other apps. If your program doesn't give back memory to the system, it's called a "leak." Leaky programs are bad; they make things crash. But it wasn't always this way.
Once upon a time, there was the oft-mocked Newton. Like the iPhone, it ran a real OS build on a robust, object-oriented language called NewtonScript. But the Newton did a major thing that the iPhone only wishes it could. It supported garbage collection, or automatic memory management, just like full-grown desktop Macs. What's the difference to you? More powerful apps.
The iPhone doesn't support garbage collection; it's not fast enough. So iPhone apps tend to "leak memory," or hang on to memory too long. Developers I've interviewed--even Apple Design Award winners--have mentioned to me that their iPhone apps are leaking memory almost constantly because they're too lazy to be really anal about manual memory management. But since most iPhone apps are relatively simple, it's not a big deal; you close the app and life goes on. (Eventually, the phone regains the leaked memory, in various ways.)
But this means iPhone apps can only get so complex before they require too much hand-tuning to be worth the time. IPhone developers have been hitting a wall: there's a lot more the device could do (at least, the 3GS, with its better hardware) but the OS is still too basic.
A tablet with faster hardware and a more potent version of OS X could run real garbage-collected apps, letting current iPhone developers imaginations run wild. The tablet app store would explode ten times as fast as the original app store; I've spoken to several developers about porting their apps to a larger format, and the consensus is that the process would be relatively painless. (The only question is about hardware-accelerated graphics; the tablet might run a different version of OpenGL-ES than the iPhone, because it will have a different graphics processor.) We'd have more than big-screen iPhone apps, though; because of garbage collection, we'd have apps on steroids.
Like other Apple products ahead of their time, the Newton was quietly axed because of low demand and scope creep. But Steve Jobs never forgets a good idea: witness the original iMac, the Apple TV, and the G4 Cube, all reminiscent of earlier failed models. Hell, the Cube even failed twice and came back to life as the Mac Mini. When the curtain lifts at Apple's January event, there may well be a Newton behind it.