People are mainly using Balsamiq to create things. The iPad isn't typically used for content creation, but for content consumption. For a UI editor, drawing tool, etc. like Balsamiq you'd be more productive with a more precise pixel pusher like a mouse rather than a fat finger. Not to say that you can't do any prototype design, while laying around on your couch with an iPad -- just that beyond a few things when you get into the details it's not that great of a form factor.