Personally, I don't mind it if somebody is talking on the phone while walking, as long as they are paying attention to where they are going. I will sometimes talk and walk at the same time, to save time, but when I do I am always even more careful than usual when crossing streets, etc. It's no more rude than listening to headphones, I think, as long as you're paying enough attention to the world around you so as to not inconvience others.
I think that texting while walking is really rude, though, because it's almost impossible, as far as I can tell, to text without looking down at your phone. If you're texting while walking, it's almost a given that you are not looking up at all times to see if you are about to run into somebody or if a car is going to run you over or what. I've seen way too many close calls when someone has been jaywalking while texting and a car has had to slam on the brakes, for example. It seems much easier to get locked into your "own little world" and to therefore cause others to have to go out of their way to watch out for you.