Then why did you call me/ come here?
Let's all pretend for a minute that it's 1998, and we've been invited to dinner. We've arrived, "Candle in the Wind" is playing on the cd player, and our lovely hosts just gave us a mojito to wash down the salsa and chips. This is nice! Just four or five people chatting.
Then, the host's phone rings. What is the "right" thing to do back in 1998? (Hint: answering machines were already common, and it's very rude to choose the person on the phone over the person standing right in front of you!)
So, listen up, if you're at a party (read: lots of people), you can certainly slip away and take a call on your cell phone in 2011. If you have a babysitter, you can do the same thing in a small group as long as you apologize to your friends. But, really, there are not that many things that cannot wait. And if it really cannot wait, then why did you come? I understand that plans change, so cancel instead of being rude.
And if you need to check your email THIS second, then please quit talking to me on the phone. I can call you later when you have time.