The Myers-Briggs personality test suggest 16 different personalities.
What percent of the US population is each personality type? Answer
must be from a reputable source eg affiliated with a university or
research group.

I found the percentages at "The Center for Applications of
Psychological Type." (CAPT)

ISTJ:	12-16%
ISFJ:	10-13%
INFJ:	2-3%
INTJ:	3-4%
ISTP:	5-7%
ISFP:	5-7%
INFP:	4-5%
INTP:	5-6%
ESTP:	5-7%
ESFP:	6-9%
ENFP:	6-8%
ENTP:	4-7%
ESTJ:	10-12%
ESFJ:	10-12%
ENFJ:	3-5%
ENTJ:	3-5%

The website also breaks these numbers down for Male and Female
Americans.
http://www.capt.org/The_MBTI_Instrument/Estimated_Frequencies.cfm

CAPT was co-founded by Isabel Briggs Myers, the author of the
Myers-Briggs Type Indicator®, and Mary H. McCaulley, Ph.D
( http://www.capt.org/About_CAPT/Home.cfm )

Presumably, these statistics are from the paper of the same name,
cited elsewhere ( http://www.capt.org/Using_Type/Education.cfm ) as
"Estimated Frequencies of the Types in the United States Population"
by Charles R. Martin, 1996
Developed by Mary H. McCaulley 
Published by Center for Applications of Psychological Type, Inc. 
2815 N.W. 13th Street, Suite 401
Gainesville, Florida 32609. 
Toll Free 800.777.2278 
Copyright 1998

(Charles R. Martin, Ph.D. is the Vice President of Research and
Development at CAPT.)


Additional Links:

An explanation of the different types can be found here.
http://www.capt.org/Using_Type/Education.cfm
 

Search Strategy:

Myers-Briggs percentage "United States"
://www.google.com/search?sourceid=navclient&q=Myers%2DBriggs+percentage+%22United+States%22

"Charles R. Martin" CAPT
://www.google.com/search?num=20&hl=en&lr=&ie=ISO-8859-1&safe=off&q=%22Charles+R.+Martin%22+CAPT

"Estimated Frequencies of the Types in the United States Population"
://www.google.com/search?sourceid=navclient&q=%22Estimated+Frequencies+of+the+Types+in+the+United+States+Population%22

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Clarification of Answer by googlebrain-ga on 02 Jul 2002 12:43 PDT

Whoops. I accidently duplicated one of my links. This is what it should have said.

An explanation of the different types can be found here. 
http://www.capt.org/Using_Type/Workplace.cfm

excellent - beyond complete. googlebrain rules!