You asked if it's possible to retain your current cell phone number
when switching mobile phone companies. You live in the United States.
Unfortunately, it is not currently possible to retain your number when
switching carriers. As the xnoise mentions in the comments, the term
is known in the industry as 'number portability.'
I did some research to see exactly when we can expect 'number
portability' to become a reality. According to a story on wired.com,
the FCC has set a deadline of November 24, 2003 for the industry to
have this implemented. The industry's lobbying association, the
Cellular Telecommunications and Internet Association (CTIA), is
fighting this requirement claiming such changes will cost carriers
hundreds of millions of dollars per year. At the same time, these
same wireless companies are asking the FCC to allow people to switch
their landline numbers to their wireless phones! Quite a
contradiction! You can check out the full article here:
So where will this story end? It's up to the courts and the FCC to
figure out a solution. The cellular companies are determined to
prevent customers from switching their numbers from one carrier to
another, so the likely result is another delay. With millions of
dollars at stake, the industry will most likely fight this requirement
well past the November deadline.
If you'd like to follow the story as it develops, a great place to
look is the Google News service. I simply typed in 'number
portability' at news.google.com, and found a number of great news
articles on the topic.
Best of luck!