Why does this article consider the iPad in the first place? As the article points out, the device is too big and unwieldy in the first place. If you were to use an apple product, an iPod Touch is the first thing that pops into my mind.
http://www.crunchgear.com/2010/04/27/the-ipad-pos-system-com...
In at least a few places in Aus (Canberra Centre, Melb Airport, Elizabeth St Melb) I've seen ipod touch sized PDA's being used to take down orders at McDonalds.. (including special orders - they got my mcflurry + caramel topping down fine).
Maccas may have a slightly limited menu - but it's still pretty large compared to some restaurants.