I have no problem with texting... except for this exact situation you bring to mind. If the person is within earshot and you can't open your mouth to say whatever it is you have to say, then it's probably not important enough to even message them with.
My main concern with texting is what it's doing to people's grammar. See this? This, right here? This is how I type. ALWAYS. Be it texting, posting, or IM, I am always as grammatically correct as I know how to be. The way I see it, "txt tlk" is not only lazy, it can even be seen as downright rude. Imagine this situation, if you will:
Say you've got a problem. A serious one. You come online and talk to a friend through an instant messenger about it. After your long, well-thought out spiel about exactly what's going on in your life and why you're in such a terrible mood, you ask for advice. All you get in return is:
"idk man"
Do you know how that comes across? The person, even after reading through your strife-ridden paragraph, still can't even bring himself to type out a response that takes more than two seconds to type. Not only is that lethargic, it also comes across as apathetic, whether they mean for it to be or not. This is a conversation, not a race. Take your time saying what you have to say, or don't even bother.
...*looks back at wall of text*... Whoops. I think I went off on an entirely different tangent.