People have the weirdest hang-ups.
It's not something I've ever sat down and thought about, really. I've had a few conversations with both friends and strangers at those trough-like urinals described earlier...not really an issue. I gives a shit if some dude's going to be checking out my dick. Whatever. I respect other people's privacy to the point where I won't go picking the urinal right next to one that's being used.
I've had telephone conversations while on the pot, though I usually apply a bit of sphincter control to ensure the convo isn't interrupted by a plop. Obviously these calls are kept as short as possible. I've never actually had someone try and talk to me while I was taking a dump though...talk about strained conversation.
So, a casual nod + "Sup?" when stepping up to a urinal is OK. Asking about the weather while I'm making a #2 is not.