With Google Glass coming I can only imagine this getting worse. People that look like they're paying attention to you will be checking their stream. You will no longer even having the privilege of knowing you are being ignored.
This reminds me of a scene from Mad Men, where Roger Sterling shares his new found wisdom with Don Draper:
Roger: You know, it's very interesting, but a lot of times you think people are looking at you, but they're not. Their mind's elsewhere. Don: Lots of people that haven't taken LSD already know that, Roger.
If you're talking with someone, rather than at them, I think it would become apparent fairly quickly that they weren't really involved in the conversation.
