The Apple tablet stands to do nothing. The "plenty of people" you refer to are the tech geeks and Apple fanbois. About 5 million people who live in the U.S. That's nothing. Its a drop in the bucket. That's exactly what Apple could count on if was an Apple Logo Turntable being released instead.
The fan base is not a market. The fan base is not a market. The fan base is not a market.
The only reason for Apple's break out success with the iPhone, as an Apple product, is that its a PHONE. One of the most popular types of devices, and one of the largest retail markets in the world. Larger than the PC which they've already struggled to make headway in.
They designed an excellent phone, and have sold a LOT. They will continue to. However. Apple did not create Mobile Phone Buyers, or the market they buy in. They were already there, desperately waiting for a great product to come along.
Where is the market for this device thats waiting for greatness to come? It's not there, And a $1000, its nothing more than a fantasy.
You are correct that a fan base alone is not a market. The product still has to be good. You are however wrong about Apple creating markets, well a much better term would be expanding on markets. Many people have switched from a "dumb phone" to a smart phone because of the iPhone. Many of those people had never even considered a smart phone before. The smart phone market is much larger now. Smart phone sales are up, other sales are down and Apple is driving that trend.
An Apple tablet falls into the computer and PMP market as much as the iPhone falls into the phone market. So there is a market for it if we use your broad definition of a market. Diving a little deeper, and closer to the true market, we are seeing the emergence of tablet devices on the mass market. Archos has a whole product line, the joojoo is coming (and is $500 for a web device), ebook readers are adding LCD touch screens, magazine publishers are readying tablet friendly digital magazines, etc. If there wasn't a potential market there, none of this would be happening. An Apple tablet would likely split that market wide open. Although I also think $1000 is a little high, it would probably still sell at that price. The iPhone sold at $600 before it was carrier subsidized, some people still pay more for one now if they are still under contract. Apple of course sold many more iPhones once the price dropped. Even if a tablet started at $1000, it could go down in price at a later date, further driving sales.
PS. You are looking at mac sales the wrong way. Apple is going quite well, considering they are a manufacturer of premium computers, not an OS distributer.