????By Philip Elmer-DeWitt

????The touchscreen tablet computer that is widely expected to be Apple’s (AAPL) answer to those $300 netbooks will cost more, come later and be more dramatically different than most investors expect.

????That’s the thrust of a note to clients posted by Piper Jaffray’s Gene Munster early Thursday.

????According to Munster, the device will fill the gap between the iPod touch and the MacBook, cost between $500 and $700, run App Store apps and arrive some time in the first half of 2010.

????“We are anticipating a new category of Apple products,” he writes, “with an operating system more robust than the iPhone’s but optimized for multi-touch…

????“We expect the end result … to be launched later but with more dramatic differentiation than the Street is expecting.”

????Making the case for a touchscreen tablet, Munster ticks off the signs:

????? Apple’s consistent message that it refuses to launch a “cheap” portable netbook

????? Its gradual addition of multi-touch technology to all of its core products (iPhones, iPods and Macs)

????? Its acquisition of P.A. Semi along with other recent chip-related hires (making it increasingly clear that Apple is investing more in its mobile computing franchise)

????? Apple’s desire to differentiate itself in a maturing market before it’s too late (similar to the timing of iPod and iPhone)

????Rumors that Steve Jobs was working on a successor to the ill-fated Newton date back to at least Sept. 2007, 18 months after a team of Apple engineers is said to have begun working on it. According to AppleInsider, the tablet team was pulled off the project in 2006 to help get the iPhone out the door.

????What’s holding it up now, according to Munster’s sources, is the operating system. “Its complexity, along with our conversations with a key company in the mobile space, leads us to believe [the new device] will not launch until calendar year 2010.”

????Munster does not address the question of whether the tablet will come with a real keyboard like the MacBook, or a virtual one like the iPod touch.

????A touchscreen tablet wouldn’t need a keyboard for videos, Web-surfing, iPhone apps or e-books. But some analysts believe it couldn’t really serve as a netbook without a physical keyboard — if only as a peripheral.

????On that other hand, it’s hard to imagine Jobs or Jonathan Ive signing off on a design as clunky as, say, the Asus t91 pictured here, with its hinged, rotating keyboard.

????Will Apple solve this dilemma? Apparently we’re going to have to wait a little longer to find out.