Well--assuming that you think it's a poor design--how would you change it so that it would be a good design?
Well one of the issues skunk's mentioned is the footprint, so I guess he'd put the thing on a stand to reduce this...er...kinda like Apple have. It's almost like they spent time thinking it through.
Like I said before the design just isn't suited to the iMac market segment. I still say a touch-screen iMac could work as an EPOS system, but this would be better served using a standard design iMac and building the desk around it.
I think a company called TrollTouch do touch-screen's for Macs, but they're not cheap. You also have to consider who would want iMac functionality in a retail environment? Not big enterprises as their staff wouldn't have any use for it, so again your market is very limited.