I don't think an Apple-branded television set will be a big success. But who knows, could be huge. I wouldn't buy one, though.
I don't know if you've heard of XBMC or Plex but they are very popular media playing interfaces which combine playing media files with live streams to create an immersive theatre like experience. The Apple TV box (the small black one) is the most popular hardware (outside of dedicated HTPC's) used for achieving this experience.
Smart TV's have attempted to capture this market but they are failing because they don't put the money and resources into making the Smart TV interface both useful and useable!!
This reminds me of the mobile phone market just before the iPhone came out. You already had lots of large screen phones that had apps (applications/programs) and many were even touch screen (via a stylus). Apple came along and all they did was dumb things down. They refined the user experience by removing the stylus from the touch screen and simplifying the apps from full programs into little more than a shell that completed a handful of functions.
The reality of the iPhone was that it was a phone with far inferior hardware than Nokia's and Sony Ericsson's of the time. I mean they didn't even have 3G which was the industry standard at that time. But what Apple did right was that it reduced the user experience down to the very basics.
The Apple apps did less, the phone had worse specs, a rubbish camera, no video calling (unlike my two year old sony ericsson) but as a package it all worked really smoothly and was so simple that toddlers could (and did) use the phone. The windows phone system of the time was really ambitious compared to iOS with browsers and office suites and all sorts of hardware, Microsoft were trying to make it into a mini PC but the software was hard to navigate and you had to wait forever for things to load up and work. You had to be a bit of a geek to understand how to add programs and how to make things work properly.
Apple said forget about all that, let's just dumb the whole experience down so a random child can use it. They did and they haven't looked back.
Getting back to the smart TV's they are exactly where smart phones were before the iPhone came out. All Apple have to do is copy the XBMC experience, restyle it within an OSX or iOS skin. Dumb it down and release it inside a TV. As long as they make the user experience their priority (as they always do) they will make a killing.
Straight after they do that, you will have all the other TV companies thinking why the hell didn't we spend more money on our user interface and features and you will have another 'new and revolutionary' iProduct that will 'change' the face of the industry. Google could have made an Android based Nexus TV long ago. Especially since they already have noted TV making companies like Samsung and Sony making Android phones for them. But no they will wait till Apple does it and then just copy them.
And @quillz you may not buy an overpriced and underspec Apple TV when it comes out but at some point down the line you will probably buy one of the TV's that will be 'inspired' by it.