I know that cell-phone use on an airplane is banned because the signal
interferes with the plane. However, why is it that at the airport,
when I am re-entering the U.S.A. through Customs, I cannot use my cell
phone while waiting in line at Customs or standing at Baggage Claim?

Hi, 

Welcome back home. 

First of all, I want a small disclaimer in here; I don't believe
anyone can explain the logic of the minds running the Homeland
security office. That said ...

--"The Homeland Security Department has barred border agents from
carrying or using cell phones or pagers at federal inspection areas
after a Democratic congresswoman complained about not being allowed to
use her own cell phone at a major airport, officials tell
WorldNetDaily.

The ban on such telecommunication devices had been in effect for
foreign travelers. Now it covers federal agents in both customs and
immigration services."--
http://www.worldnetdaily.com/news/article.asp?ARTICLE_ID=32653

Cell phones are getting a great deal of bad press from the security
community, and if you think about it, this does make some sense. Cell
phones are not just phones. Many of them are small offices. They can
be used for several nefarious purposes, and can "hide" hidden
abilities as well. Let's look at some examples:

Cell Gunphone Discovered
http://cellular.co.za/phones/gunphone/gun-phone.htm

Yep, what you are think is exactly what that article is about. 
--"At first sight it looks like a regular cell phone ? same size, same
shape, same overall appearance.

But beneath the digital face lies a .22-caliber pistol, a phone gun
capable of firing four rounds in quick succession with a touch of the
otherwise standard keypad.

The US Department of Homeland Security and the FBI are aware of the
device and have instructed baggage screeners to be on the lookout for
suspicious mobile phones."

FBI warns of cell phones aloft
http://news.com.com/FBI+warns+of+cell+phones+aloft/2100-1039_3-5726850.html

We know about the dangers of cell phones and guidance systems, but the
FBI doesn't like them at all, and wants to make sure they can wire-tap
any conversation on an airplane.

While that is on the plane, the same concerns are seen to be a danger
inside the airport as well. Cell phones have not only been used to
coordinate terrorist activity in the past, but to also remotely
detonate bombs, take photo and video images of security locations, and
record security interviews.

Until you are out of the baggage claim area, you are still in the
security control area of the air port. Again this makes perfect sense.
Baggage claim is an access point to the inside of the airplanes, and
other peoples baggage. You can't ignore the fact that there are not
enough security personnel in these areas to over come a couple of guys
with cell phones teaming up a distraction ruse to slip something into
a bag, or remove something from a bag. On top of this, its still an
acceptable target for angry people to take advantage of.

Other articles of Interest

Cell Phone Hearing
http://www.house.gov/transportation/press/press2005/release88.html

Debate over Disabling Cell Phones
http://www.cbsnews.com/stories/2005/07/12/national/main708617.shtml

The Best Security Doesn't Always Begin at Home
http://www.sun.com/br/1204_ezine/gov_security.html

thanks, 

webadept-ga

Answer effectively addresses reasons behind the cell-phone ban.