This bit of soothsaying is courtesy of Forrester Research, which has just published a on how Amazon will quickly become Apple's biggest competitor in the tablet space.

Amazon reportedly is readying an Android tablet that may cost "" than the iPad, and may launch the product this fall, according to recent rumors. As Forrester sees it, an Amazon-branded slate priced below $300 would be a huge hit--selling 3- to 5-million units in the fourth quarter alone, assuming Amazon has enough supply to meet demand.

Amazon's tablet success won't challenge the iPad's dominance, however. Forrester analyst Sarah Rotman Epps, in a blog post, points out that Apple has sold nearly 29 million tablets thus far and "looks invincible."

But Amazon's retailing prowess will allow it to gain a sizable chunk of the market quickly, something other tablet makers have yet to accomplish.