Have you been in the middle of a conversation and have a third person walk up and ask a question like you werent even there?

Common courtesy is increasingly uncommon, LTC (Join to see).
Generally if I need something, I will wait for a pause in the conversation. If I have to break in, a simple "excuse me" should suffice.
Just breaking in is rude, no matter who you are.

I have two problems with that scenario: one, it does happen a lot. The arriving third party bothers me because they must surely see a conversation is taking place. Secondly, and oftentimes forgotten in this equation, is the person to whom the interrupter is addressing their interruption. When the person with whom I HAD been having a conversation, knowingly engages the third party, I'm now pissed at TWO people. I'd have to say that people, in general, have long forgotten the simple rules of etiquette when it comes to communication. Back to my example, if the person to whom I was speaking does not take the bull by the horns and tell the third party to excuse them for just a moment while he or she and I finish our chat, I WILL DO IT FOR THEM and say, "Excuse you, but we were talking!!!"

No it's not our culture changing. It is just that some people are self important assholes, and what they have to say and do is more important than anyone else. They have no manners or respect for other people. They usually see everyone else as beneath them.