I will be travelling to the USA this week for 16 days and would like a
prepaid mobile phone.  I would like to pick it up at Phoenix airport. 
Can you suggest anyone.

You can go for the Cingular or Tmobile Go-phones which are available
at cingular or tmobile stores or leading retailers like
Walmart,bestbuy. The store personnel can help you to activate the
phone.

http://www.walmart.com/catalog/product.do?product_id=4183899

http://www.bestbuy.com/site/olspage.jsp?id=pcmcat1200050009&type=category&_DARGS=/site/en_US/catalog/fragments/product/olslinelistingsortfilter.jsp

If unable to view the above link goto www.walmart.com or
www.bestbuy.com and search for Pay as you go or Go phones.

I personally used Tmobile's service when I was in the USA.  I carry a
GSM handset back in Singapore and its Triband.  I had no problems
using their GSM sim card after a short application and swapping of the
chip needed.

I personally opted for their monthly prepaid plan, while it Cost me
49.99 a month (a midrange goPhone plan), I felt that the free
nighttime and weekend minutes really helped.  I had used the pay as
you go service the year before and chalked up a much hefftier bill. 
If you plan on calling your friend/associates during the
evenings/weekends I would highly suggest this over another plan.  I
had used the phone to talk to friends on average an hour a night with
the payasyougo plan and had chalked up a 100 dollar bill over 20 days

You can check out http://www.t-mobile.com/prepaid/retail.asp for a
retail location at the airport.  The best bet however would be next to
a downtown hotel/shopping area.