Suppose you are at U.S. Customs in the airport, and you enjoy using
your cell phone. What punishment awaits you if you repeatedly ignore
personnel verbal warnings to turn off your cell phone, and your phone
does not stop ringing (because people are calling you)? Will your cell
phone be confiscated? Will you be detained? Imprisoned?

Better not to upset Customs anywhere.

Boy, can they get nasty!

They can turn all your bags inside out and make you strip.

In Rio, I once saw a guy arrested who tried to be smart.

"you enjoy using your cell phone"  -- in that case your life is so
empty and meaningless that prison (and that full cavity search) will
actually improve your social life.

A cell phone is a convenience, to be used at appropriate times.

Prison labor imports have been a violation of US customs law since
1890 under the McKinley Tariff Act [19 USC, section 1307]. Why not
tell the agents to arrest China, Jordan, Pakistan, and a few other
allies before bothering people with piddling issues.

Oh, wait-- piddling issues is what our nation is turning towards
solving, 'cuz it's so easy.

If you're at U.S. Customs, ignoring warnings to turn off your cell
phone, then you probably have a death wish or are incredibly stupid.

Cell phones have been identified as being able to trigger/detonate
bombs (I think Bali is an example).

I think losing your cell phone would be the least of your worries.

There exist people that try to be "smart" and disobey rules. I've also
heard of a few people that managed to sneak past the U.S. Customs with
their cell phone on vibrate or hidden away where nobody hears it ring.
I don't endorse any of this, and it's clearly not legal. I'm just
curious to know what is the punishment if Customs catches you and you
ignore their warnings, but you don't have any illegal weapons or
bombs. I.e., how strict are they, considering that 99.9% of cell phone
use is for harmless matters such as chatting with your friends?

Certain things have a definite punishment. If you're caught with a
cell phone in a courtroom, it is confiscated, and you don't get it
back, and possibly pay a fine. If you park in a non-parking zone, you
pay a fine and possibly have your car towed. If you drive drunk, the
penalty for a first-time offense is usually a small fine, community
service, attendence at drunk-driving school, and treatment for
substance abuse. For subsequent offenses, it is a large fine,
driver-license suspension, and a mandatory jail or prison sentence.

All of these things are clearly stated in the law if you look hard
enough. What is the penalty for cell-phone use at U.S. Customs? I
can't find it anywhere. I've searched the web and came out empty,
except for a site from 2002 claiming that all they do is confiscate
the phone, but that rule may have changed since then. (BTW, unless I
am mistaken, more people died last year from drunk-driving accidents
than cell phone bombs.)

The offence might be more along the lines of "Failing to follow
directions of Customs Officer" etc.

Might be a security issue with photos being taken inside customs also.