Common courtesy isn't that common.
I feel like it's fine to talk on your cell phone. I think they should be illegal while driving, unless it's hands free. Like many have stated, it's easy to blame cells since there are worse distractions in the car but if cells were illegal it would be one less distraction to have.
I use my phone, mostly to talk to my sister who lives in NYC. However, I don't talk while driving or to take time away when I'm doing something with my kids. Sometimes I chat with her at the grocery store, it's almost like she's with me. While I'm shopping and talking, I still say "excuse me" when I walk infront of someone studying the shelves. I also put the phone down when it's time for me to pay. I give the checker the courtesy of my attention, a smile and a "thank you." I think you can still have manners with a cell and there are plenty of people who are still jerks without them.
I think there are many restaurants starting to ban cell phones. I'm not all that bothered by them but with two young kids, it's not like we have many chances to go out.
When we do, I will take my cell phone, in case there's an emergency with the babysitter but I'll also have the courtesy of putting the phone on vibrate.
I think what this boils down to is that people are people. Where ever you are there are bound to be nerds, airheads, cute kids, jerks, crazies and everyone else. My husband always notices when the house is dirty but never when it's clean. Maybe all we notice anymore are the people who are rude and not the ones who are nice.
Whew! I need a pick me up after reading this thread. I think I'll go look at our sleeping kids.