(CNN Money: How Apple Became a Monopsonist)
OLED technology is notorious for being difficult and expensive to produce on a large scale. Few suppliers are willing to risk the cost of production without knowing they'll have buyers. Therefore, there's little investment and a lot of room to grow in OLED manufacturing technology. The speculation is that Apple will be that investor, paying in advance for a large scale production of large OLED TVs, and thus securing the market from imitators.
Apple has shown a reluctance to adopt OLED displays in its devices. Whereas OLEDs have been implemented in Android phones and tablets, the iPhone, iPad, and even the iPod Nano still uses TFT. But these are all multitouch displays, including the Nano, with Apple's characteristic high fidelity response to touch, which is not cheap. An Apple TV display would not need any kind of touch interface.
Plus, the benefits of OLED are not as appreciated on mobile devices. When exposed to direct sunlight, the high contrast that OLEDs deliver are washed away. They are most appreciated on a TV. I can speak from experience, since I own two (the Sony XEL-1 and the LG 15EL9500) and they're phenomenal in a darkened room displaying higher res content. The future of mobile displays should really be something closer to e-Ink or maybe even transparent OLEDs.
In any case I consider this speculation believable if not very likely. If true, we won't even see a release until 2013 or late 2012 at the very earliest, which is when LG says it intends to start manufacturing 55" OLEDs. No other company has more invested in OLED right now than LG and Apple has not payed more in advance to any other display company than LG. Speculation was that this was for OLED displays in the iPad 2 and that was wrong.
What we know, though, is that, if Apple is going to release a TV, it will want it to be the best-looking display on the market, and that means OLED. It may be that OLEDs are not ideal for mobile devices or even computer displays (the high contrast can actually strain the eyes a bit when reading text, causing a blooming effect that's purely biological) but moving pictures are much more suited.