It's hard to say because I know several iPhone owners who don't follow the iPhone news like we do here and they also don't sync with iTunes very often. That said, if they all received the text message about the update, they might be inclined to update. Yeah, 50% seems as good a guess as any.
I got a replacement iPhone 3G (16GB) Sunday evening. The genius told me it would likely need updating to 2.1 when I get home, but we were both surprised to find it already had 2.1 on it out of the box (new from inventory, not refurbished).