As is the norm for most rumored Apple products... it's been done before to some extent. In this case, a company in Valencia, CA, humorously called
Troll Touch (wonder if they hang out on forums much?) performs touchscreen upgrades to virtually any Mac product. As I understand it, the majority of their customers are
businesses that use touchscreen iMacs as point of sale machines, equipped with the [/url="http://www.xsilva.com"]Lightspeed POS[/url] software. And no, POS does not stand for what you call your '79 Ford Pinto.
I could see there being a strong market for systems like these, especially considering the markup Apple could put on them. I've worked heavily with Radiant POS systems which run XP Embedded and used a few other XP Pro software packs and CE based devices - most of which crashed on a weekly basis. When running a retail/restaurant business, the last thing you want is to lost precious time and customers due to a faulty system. Macs have a great reputation with not crashing, so I can see why a smart business owner would see the advantage of investing in a few of these.
As for incorporating iOS in there, I could see it being a very light "embedded" version of OS X that would be great for running on smaller, less powerful computers - much like XP Embedded. Kind of like what it is already, except capable of being run on other hardware platforms.
And finally, I could see an iMac-based Surface-like Mac. You know, tabletop computing and all. Not as much market, but pretty dang cool, even if Microsoft has done it before. We all know Apple's always trying to one-up Microsoft.
Be as it may though, I couldn't see Apple pumping out a consumer-level touchscreen iMac. For anything other than light, 15 minute usage, it would be a real pain. I couldn't see many non-business owners buying one. I'm putting my money (har har) on seeing an Apple produced iMac POS either running OS X or a slightly beefed up form of iOS. What do you think about the name OS X$?