This would be an interesting device, if they did it, and I wonder what model they'd use for software? Would all Mac OS X applications just run, or would they try to use an AppStore model with a new SDK designed specifically for it? After all, a device like that would have limited screen real estate, an accelerometer and a virtual keyboard... not many main stream Mac applications would be pleasantly shoe-horned into it. iPhone applications might assume a specific screen size, and might not look so great on a much larger screen.
Apple doesn't seem to make many niche products these days (Apple TV seems to be the exception), with their very clearly segmented Mac products, iPods and iPhones.
Still, I'd buy one just for web access at home while watching TV or lying in bed. Imagine if it could read Kindle books, too... could be cool.