Barnes & Noble's Nook eReader is becoming a more tablet-like device with an update to Froyo, better web browsing, email and third party apps. The 7" eReader is already a success but it could become one of the most successful Android tablets because of one particular feature -- it's $249 price tag.

General consumer audiences will be reluctant to spend $499, the entry level iPad price, on smaller (and otherwise inferior) tablets. The 7" Playbook, for example, starts at $499 and the Motorola Xoom was a flop in part because of its higher entry level price tag. 

However the $249 price tag for Nook is incredibly attractive. And as Nook becomes a more full-fledged tablet more people will buy one. The Nook will continue to represent itself as an "eReader" and not a tablet; it's an eReader with other features that just happen to include web (w/Flash), email and apps. 

Many people have speculated about Amazon eventually offering a full-blown Android tablet. Bet on it. This only makes it more likely.

Amazon will need to respond to the Nook's challenge on the eReader front and so the next-gen Kindle will probably have to offer color, apps and a better overall web experience. But to really "hit it out of the park" -- unless Amazon is going to directly compete with computer OEMs at the high-end, which I doubt -- the Amazon tablet will need to be smaller than 9" and cost less than $300 (say $299). 

The long-term threat to the iPad, just like the iPhone, comes from an army of cheap, "good enough" devices featuring Android. Two of those could come from Amazon and Barnes & Noble.