Is it rude to talk on a cell phone in all public places or just in theaters and restaurants?

Yes, yes, and yes. Again, a favored and current reference is 21st Century Etiquette, A Guide to Manners in the Modern Age. The author made a good point by asking the reader if they would "take their alarm clock out in public". That is just the ringing problem, the talking is worse.

The author's advice is common sense when it comes to public cell phone use:

Do. . .

-Respect those around you when using your phone
-Keep your voice down
-Take calls in private or in a less crowded area
-Keep conversations short
-Turn your phone off or switch to vibrate

Don't. . .

-Try to talk to other people (like a waiter, store clerk, et cetera) while you are taking a call
-Discuss intimate or private matters
-Use foul language
-Talk about other people by name
-Have lengthy conversations

Personally, I can't stand these yap fests at the grocery store. They block the aisle and don't move along because their attention is diverted. And the constant yapping makes it hard for anyone in their midst to remember that gallon of milk. If they have kids and a phone. . .don't get me started. I digress--hope this answers your question

UPDATE:

Poll Finds Public Cell Phone Use Annoys Most
(source) AOL News Service

WASHINGTON (March 14 2005) - A majority of cell phone users acknowledge they get irritated at other people for using cell phones in a public place.

The University of Michigan poll found that six in 10 cell phone users say that using a cell phone in public can be "a major irritation."

About four in 10 said there should a law that prohibits people from talking on cell phones in public places like museums, movie theaters and restaurants. Eight in 10 said cell phones are a major safety hazard if used while driving.

But cell phone users also like the convenience that cell phones can bring.

About eight in 10 cell phone users said the phones have made their lives easier. By comparison, six in 10 e-mail users said e-mail has made their lives easier.

"These findings suggest Americans have mixed feelings about cell phone use," said Michael Traugott, a researcher at the University of Michigan's Institute for Social Research.

More than half said they liked all the information that is available to them because of new technology, while about one in 10 said they felt overwhelmed.

The poll was released Monday at the start of the annual meeting of the Cellular Telecommunications and Internet Association in New Orleans.

The poll of 752 adults was taken from March 3-10 and has a margin of sampling error of plus or minus 3 percentage points.

Some people have the decency to carry on a brief, quiet conversation. Most people, however, bellow at the top of their lungs. It is the new exhibitionism.

ah it depends. I mean if i'm at Best Buy i dont think anyone would mind if i was on the phone real quick but not somewhere where i would be disturbing to others...

Just as long as a person with a cellphone is NOT talking too LOUD.

In a restaurant possibly. Recently my wife and I were in a nice retaurant when a young couple were seated at the table next to us.
He was saying something, she was scrolling her blackberry. To me she was saying loud and clear, feed me, pay the bill and take me home so I can do something that interests me. It's really not too different than the guy that takes a pretty girl out to eat and the dope sits there looking at his watch.

In a theater; I love the story I read this last week about an actor in a play who stopped the performance, turned toward the audience and said to the person who's cell was ringing,
"Shouldn't you take that?"

Strolling through the grocery store, Wal-Mart or Home Depot is another story but don't stand in the middle of the aisle to do it. Stay out of the way.

Its rude when you are checking out of the store,it is rude to answer it without excusing yourself from the conversation at hand!~

I think just in theaters.

It's also highly inappropriate to have your phone ringing (or be chattering on it) at church or the library.

Yes, rude and annoying.

I think just in theaters.

I don't want to hear someones conversation in a movie theatre or restaurant. Years ago, people would step outside to take a call or go the restroom. Privacy isn't something people are aware of until theirs has been invaded.