As an anecdotal point of reference, I monitor a number of stackoverflow categories with RSS feeds. I commonly read and answer these feeds with Reeder on my iPad sitting in a comfy chair in my living room, not at my computer.

I answer less questions than I normally would if the interface were better.

(maybe this is fitting into the definition of "a few hardcores" for you though)

There are many questions that I don't answer, because I'm on my iPad, and it's way too much of a pain to bother.

You're also focused on the users of SO specifically. There are Area 51 sites that would be more prone to having users on mobile devices.