Guilty. But I only do it after the conversation has really died down and we haven't said anything for about 20 minutes, and it feels almost acceptable to leave without saying anything. Yeah, it would definitely be more polite to just officially say "Bye", but with me and my friends, we really only do that if we've been keeping up a conversation without too many pauses and one of us suddenly has to leave.
I think it's become the norm for a lot of people, as I'm pretty sure my friends do it to most of their other friends, too. I know what you mean, though.