If they feel threatened enough by Circles, I have no doubt that they will make it more visible, and available on all platforms.

That said, it's surprising how few people, even tech savvy ones seem to know about the functionality even though it's been around for ages. In fact, FB used to have a "how do you know this person" question when you added a friend, and they confirmed the relationship when they accepted, which is gone. It could be that they believe that most users don't want to segment their friends by categorizing them.

The iPhone client handles groups to an extent. You can pick one group to see a status update, but can't pick multiple groups, exclusions or custom on-the-fly groups

true. apparently windows phone has a concept of groups too but then that does not sync with the lists that you have on facebook.