As usual, the provenance of the rumors surrounding the appearance of an Apple TV set is the component suppliers in Asia. Bottom line: if the suppliers can't supply the components, the TVs won't be available. Apple apparently wants to offer a state-of-the-art OLED TV, like the ones shown by LG and Samsung at the Consumer Electronics Show. But to offer such a TV, you need to have the parts. According to a report on DigiTimes, Apple is intending to use Sharp as its supplier for the panels used in the OLED displays, but Sharp isn't yet ready to ship these panels. Plus, they're still having problems with the yield rates — meaning that too large a percentage of those made have defects that render them unusable. It greatly increases the costs if you have to scrap a large number of the panels that are being made.
The report says that LG and Samsung are also manufacturing the panels that Apple needs, but they're not about to sell them to Apple, since they need them for their own OLED TVs that they plan to bring to market this year. This is one reason why Apple tends to invest in the supply chain in Asia: so that it can lock in a reliable supply of components. In addition, once you lock in supply and have a large volume, your costs are reduced.
AopleInsider gives a good overview of the situation, and notes again Apple's investment last fall in Sharp. And if you're curious about OLED technology and the IGZO panels that Apple from Sharp that Apple is expected to use in its TVs and handheld devices, check out this helpful article on PCmag.com. The article is somewhat dated, in that it doubts that manufacturers will ever exclusively move to OLED. The indications from Apple seem to be that OLED is in the future of all of its devices. The colors are richer, the technology is thinner, and it uses less power.
So the upshot is that the rumors of an Apple TV being introduced this spring are starting to look unlikely. On the other hand, the rumors do continue to suggest that an Apple TV set is in the works and will appear at some point. Hopefully later this year. Note, though, that DigiTimes has an uneven track record regarding their rumors, so don't put a lot of stock on this.
And another tidbit from DigiTimes is that Apple will be unveiling the iPhone 5 in June at the Worldwide Developers Conference. DigiTimes' source is Daiwa Securities, as quoted in a Chinese publication. Again, there's no way of knowing how accurate this info is, but it makes sense, given that Apple has often introduced a new iPhone at this event, with last year's October introduction of the iPhone 4S being an exception. In the case of the iPhone 4S, there was speculation that the delay was due to constrained supplies of components. Which again suggests that there are factors beyond Apple's control and that the appearance, or lack thereof, of an anticipated new product often depends on suppliers.