HP will certainly sell some units, but Apple's approach to build a consumption-oriented device with an advanced, purpose-built touch interface OS with (we hope) great battery life is probably one that will bring it tremendous success.
The iPad leverages/improves upon the great experience most have had with their iPhone/Touch (which in many ways were also a "test" for Apple to see how much people would embrace their concept of touch computing). With great desktop OS app synchronization, .Mac for cloud synchronization, Apple, as usual, has built a compelling set of overlapping products/technologies that combined make it difficult for competitors to match, because they invariably can focus on one or a few elements (ie, HP can do their best to build a great tablet, but they are stuck using someone's else's software, limited by not being able to control or optimize core functionality or capability. and left to the whims of Microsoft to fix bus, inadvertently introduce new ones, or change methodologies which might reduce stability, performance, or positive user experiences.
The build-out of Apple Stores also was incredibly important, not just to showcase and sell product, but to provide a strong, easily accessible support infrastructure for their growing line of consumer products - again, this is unmatched in the industry.
Google and Microsoft are be the long term competitors, since they are "strong" on the software side (jokes/realities aside), but they have the disadvantage of not having decades of experience designing and building HW systems (often with custom chips to optimize performance). But if they choose to invest heavily, they have the best chance of putting heat on Apple. Then again, Apple will have some long lead times
Apple's long term, product planning, and market stratgies should be a model for other companies. Even releasing the iPad one day before Easter, where invariably one excited iPad owner will bring the device to a family gathering to show others, is sublime as it is obvious.
Above it all, though, Kubrick's got one up on Apple - doesn't his tablet look an awful lot like a 16x9 screen?