Blackberries and smartphones are everywhere, prompting people to question their use in meetings, dinner dates, and church services. Different modes of communication have us choosing among whom to respond to: the phone ringing, the IM pinging you, the spouse chatting in your ear. Without sounding like a prude, I'd like to offer some general guidelines on how not to offend. Granted, there is a class of people who have to be available at all times for emergencies; but I think many people take their digital contact further than they have to. Here are some general rules:

1. People you are in the room with get your attention first. If you interrupt an in-person conversation to use your Blackberry, you are signalling that the person on your phone is more important than your in-person companion. There are times when a phone call IS important, such as a call from the babysitter or the agent saying you got the part, but if you have to answer a phone call, you need to judge whether the person you are with will accept being temporarily discounted.
2. Instant Messages have a protocol: People are terrible at setting their status messages (me included), so there is usually a first query (a ping) asking if you are there. If you answer, the expectation is generally that you will have a real-time conversation for a period of time. If you are going to step away for ten minutes in the middle of a conversation, you generally need to say so. Saying good-bye varies. Some chat conversations have a good-bye and others simply "fade out."
3. Phone: Phone calls should be returned the same day, when possible, but keep in mind the first guideline about not interrupting in-person conversations.
4. Email: Email should be answered wthn 24-48 hours. If you are away on vacation, set an automated response so that people will know when to hear from you.
5. Letters: Do people still get these? Yes. It depends on the context of the letter, but personal correspondence (RSVP's, letters from your niece) should be answered within a week, perhaps longer if it is a group response, such as a long list of thank-you notes.

Again, these are general guidelines. Interrupting someone sends a signal they are less important than who you are interrupting for. Use sparingly. I don't mind if I'm having lunch with a girlfriend and the babysitter calls, but I do mind of she chooses to catch-up with another friend during the time that we set aside for each other. I would understand, if at a job interview, the interviewer gets an important phone call, but I do mind if we are interrupted constantly because it means that he or she didn't pay me the courtesy of setting aside time to talk to me.

Never underestimate how far respect will get you.