The Apple tablet has gone from a rumored device to a probably reality. Now that it's almost certainly going to arrive (though when is another matter), there are two companies that should be extremely concerned with the product: Amazon and A&T.
Apple is supposedly talking to book publishers about electronic books that would sold via iTunes. This would slaughter Amazon's line of Kindle readers. Technically, the Kindle offers a superior reading experience thanks to e-ink technology, but Apple and iPhone are much stronger brands when it comes to consumer electronics. Even though the Kindle has a significant head start, Amazon can't match Apple's experience at marketing consumer electronics. Similar to how there were plenty of MP3 players before the iPod and plenty of smartphones before the iPhone, the Apple tablet has a strong chance at leaving other e-readers in the dust.
The less obvious threat is AT&T. The Apple tablet and the rumored iPod Touch update should have the company concerned for two reasons. The first is a smaller, short-term concern; there's a small subsection of iPhone users that would gladly drop the iPhone with AT&T for an Apple tablet and a phone that excels at making phone calls (a crazy concept, I know). These are mostly customers in
real highly-populated cities -- places where WiFi can be found ever couple of yards and AT&T's service is the weakest. Using Skype with an Apple tablet or a new iPod Touch is enough for these people's communication needs. (Heck, in the last month I spoke with two gaming executives that use Skype on their iPhones over AT&T, simply because the call quality is much better.) Throw in a svelte handset and these customers are totally covered.
As for the larger threat to AT&T....