Just as emails are taking the place of snail mail, landlines are being replaced by cell phones. Most college dorms don't even have landlines and I bet 9 out of 10 20something year old apartments don't have landlines either. The mobility the cell phone allows means your personal and private conversations happen whenever and wherever. Since cell phones have become a norm, there have been accepted practices that are considered to be proper etiquette.
When to Use: Always be aware of your surrounds when your about to use your phone. Its not appropriate to take phone calls when your interacting with other people. Even sending and reading text while interacting with other people is considered rude. You are making the people you are with feel insignificant and not as worthy of your time. Be sure you keep your phone in your pocket or in your bag when appropriate. Use common sense for this. There is nothing more disrespectful than using an cell phone at an inappropriate time!
Keep it Private: Do everyone your with a favor and keep your private conversations that, private.
Texting: Texting is similar to emails in that they can always be stored and can always be forwarded. Make sure whatever you are texting is something you would be OK if others saw. Also don't get lazy with your texts. No, it shouldn't be as long or as formal as an email, but try to avoid texting with too many acronyms.
Emailing via Smart phones: Everyone has smart phones, which gives emails literally at your finger tips. Same email rules apply as if you were typing out an email on a computer! Don't get lazy!
Never too late to Right a Wrong: If you are guilty of having your phone out during inappropriate times, start changing that now. Start interacting with the real world!