Re: TiVO - they make a good product that most users prefer over whatever lame DVR box their cable company gives them. Their UI is great, and simple enough that most non-techies can pick it up easily (my wife is a TiVo master). The problem is getting them into the hands of people in the first place *most folks (like, for example, my parents) just accept whatever the cable company rents them and don't see a need to pay for an extra monthly service. They're also hobbled by their price, constant product churn (my TiVoHD is already "old") and poor market penetration.
ATV2: would be great if they would just open up to more content providers (Hulu, Amazon, Pandora, Roku-like channels) and allow more UPnP connectivity like streaming music from a NAS without iTunes. There's so much more a dedicated box like this could be doing, but Apple just doesn't see the need to innovate. No wonder so many peeps jailbreak these things. As it is though, I like using mine for Netflix (best version of their service, IMHO) and we watch Youtube a lot on it. AirPlay is pretty rad.
An actual Apple TV: would be the dumbest move ever. The TV industry is heavily competitive with low margins, and I don't see how they could compete with other established brands unless they offer something truly radical. Like... well I don't know. What's left to innovate with TV sets? They have 3D now, plus links to social networking, Netflix, web stuff etc. Apple is also known for their over-priced display monitors so I don't see them being able to produce anything at a low enough price to compete. I personally wouldn't care about an Apple-branded LCD, even if it did have an ATV3 built in. Show me 1 or 2 cool things it can do that my Sharp Aquos can't and maybe I'll reconsider...
Set top boxes in general: will never truly take off until the general public is willing to radically shift the way they view content on their televisions, and if the content providers can figure out how to integrate streaming, rentals and downloads into their business model. A lot of folks still watch cable TV *live, no DVR, with commercials and the cable providers are happy to oblige them. There's no ala carte, and it's no cheaper to rent more than a handful of shows from any service at this point. There are multiple ways to watch a TV show, from different providers with different costs. It's all fractured -- and that's how this market will remain until forces dictate otherwise.