I frequently read on my device(s) in public. Like many have said, I get "Is that a Kindle?" a lot.
I also get, "Is that one of those...you know..." *hands waving about excitedly*
Me: "Ebook reader?"
They: "YES! One of those!"
Once someone asked me if it was an electronic Bible. I'm not sure what that is. Perhaps they see people at church with the Bible on ebook readers? Or perhaps PDAs.
I am usually happy to tell curious onlookers about my devices and answer questions. I rarely get negative feedback. Either they keep it to themselves or they were interested anyway if they asked a total stranger about it. And I tell everyone to visit mobileread.com (among other sites).