Despite what Steve Jobs said 3 years ago about touchscreens being terrible on vertical displays, I think it is absolutely inevitable that Macs will sooner or later be getting touchscreens. After all Steve has been been wrong/lied before (iPod video, 7" tablet, larger iPhone screen, etc.).
Touchscreens are becoming the norm. This is the case with iPad and other tablets. This is the case with Windows 8 laptops, all-in-ones, and convertables. Hell even Chrome OS seems to be going all-in with touchscreen. By the end of the year the marketplace will be completely flooded with them all. The "phantom touchscreen" effect, where you feel like (or sometimes actually do) reach out to touch your screen out of second nature, will only continue to intensify. Mac's will start to fall behind on perception, because all other top class laptops will have it.
Then of course just look at the iOSification of OS X. Sure it's not nearly as touch friendly as iOS or Windows 8 metro, but there certainly are a lot of touchable elements. I don't think touch will be the only input method, but it suppliments the trackpad well.
Do you agree that it's just inevitable at this point? When do you expect it? It obviously won't happen till Apple introduces a much more touch friendly interface. I don't expect it in 10.9, but possibly next year? Maybe that's what Jony is working on...
Ow! Steve was right!
My wrist screamed in pain at just the THOUGHT of spending all day reaching towards a 27" vertical display! No, it is not just resistance to something new, it is idiotic from an ergonomics standpoint. Here I thought nothing could be worse than a mouse ...
Okay, so what if you would go with a horizontal display? I see several problems offhand:
1) People are used to computing desktops being vertical. While horizontal may seem more normal you have to overcome the image rather than doing it by fiat.
2) I do not know about anyone else, but any desk I have ever seen quickly becomes covered in papers, books, and what not. A flat surface will attract stuff so that is something to look at as well.
3) A horizontal screen will get dirty fast. Hands (and arms) will naturally rest on it, transferring oil, and dust will fall on it. To say nothing of coffee spills and the like.
4) If you think the ergonomics of reaching to a vertical screen are bad for your wrist think of them for your neck looking down the whole day!
Basically, I see tons of disadvantages. No real potential advantages other than it being something new. The reason they do for phones and pads is because they make sense there: you are looking downish already (and can adjust the angle as needed), are not using it for the whole day, and it is a smaller area that will not fit a keyboard.