Verticle markets were a hypothesis about how to get people to pay. Tablets were actually used in the 1990's in specialist/niche applications. The combination of enterprise customers with special needs would fork out $$$ for special-needs units. This actually did happen -- look at pictures of the NYSE trading floor from the time. People with a lot of money (and more to be made) underwrote secial-use tablets (complete with wireless 28.8 modems, etc)
Today, an iPad likely does not need modification to do most tasks, that were the placeholders discussed in the 1990's. It would have to be 10x better than the iPad, to be compelling to a vertical market player today. But, I don't think there is an order of magnitude opportunity on the sidewalk. The main constrains seem to be power/weight. As soon as you get to 2x Ipad4 you are looking at something like an 11-in MBAir, but that's already on the market for 1.5x the iPad cost.
So a speciailst unit would need to be something like the size of the iPad mini but with the power of a MB air. All in some kind of form factor (hardened/waterproof/secure) that was unique to a specialist application. I do think there are opportunities for that, but I'm inclined to think AAPL might be better to have a go at them. They would need an OSX competitive operating system, something like 512GB of flash memory, and some unique specialist application SW to be written.
That seems like a lot for a new entrant to pursue, and its unlear the market is large enough for a new entrant. Where the consumer/global market is now 1 billion middle-class professionals, gov'ts, and schoolkids, etc. The issue with pricing these things at $4-6K is that the depreciation would probably be huge.