Gartner - seriously??? Historically as accurate as S&P, particularly with regards to the mobile market.
But overall, I agree - the iPad lead is unsustainable, and falling component costs mean that someone will be able to produce an effective web browsing tablet at a fraction of the cost.
The elephant in the room, though, is software. f Apple had launched the iPad without first building the iOS development ecosystem with the iPod and iPhone, I suspect it would have been an expensive flop, or niche success. Like WebOS right now, it would have been trapped in the cycle of low of software due to low sales.
Android, of course, has massive device sales - but Google are still struggling to build a software market that can sustain a comparable level of commercial development to Apple (and like Apple's strategic errors, this one is caused by Google's own bias against paid content).
And that is what is stuffing Android tablets - a lack of 'killer' software, like Garageband or The Elements - and it is going to take a long time before device numbers become large enough to incidentally create a large enough market.
That's Apple's real runaway lead.