Are the members of the U.S. Congress exempted from insider trading
rules, where they are allowed to profit from information learned from
governmental sources, such as pending government purchases, or pending
drug approvals, et cetera, where if a regular citizen does so profit,
they could be prosecuted?

The answer is no.  Members of the United States Congress are subject
to the same insider-trading rules as are ordinary citizens.  In fact,
a member of Congress, Senate Majority Leader Bill Frist, is currently
being investigated for insider-trading in HCA Inc. stock by the SEC.

"Feds investigating Frist's HCA stock sale" Associated Press, MSNBC
(September 23, 2005) http://www.msnbc.msn.com/id/9450770/

"The Wall Street Journal reported Friday that the SEC is looking into
whether Frist had any inside knowledge that prompted his sales."

"Frist's HCA Stock Sale: Insider Trading?" by Stephen Bainbridge,
ProfessorBainbridge.com (September 21, 2005) 
http://www.professorbainbridge.com/2005/09/frists_trades.html

"Insider trading is trading while in possession of material nonpublic
information. Although the sale was effected by a trustee on bahelf of
a blind trust, Frist could still face insider trading liability
because he is the trust beneficiary and retained the power to direct
the sale of trust assets."

"Documents: Frist knew contents of blind trust" Associated Press,
CNN.com (2005) http://www.cnn.com/2005/POLITICS/09/26/frist.blindtrust.ap/

"A possible presidential contender in 2008, Frist now faces dual
investigations by the U.S. attorney for the Southern District of New
York and the Securities and Exchange Commission into his stock sales."

"Frist denies wrongdoing in stock sale" Associated Press, CNN.com
(2005) http://www.cnn.com/2005/POLITICS/09/26/frist.stocksale.ap/

"The Justice Department and Securities and Exchange Commission are
looking into the sales."

Sincerely,

Wonko

Search terms: Frist insider-trading

Decades ago, when I asked an attorney about insider trading, he said
that there was a rule-of-thumb called "the chinese wall" where anyone
working for the company in a decision-making capacity was on the side
that had to be watched. One might think that a UPS driver would be
privy to information about the success of the company by monitoring
how much business they were doing, but he wouldn't know if they were
about to go into debt, merge, etc.

This observation, too, might apply to the example of a single
contractor, even if it were a US government official.

You will likely get a full answer given the $50 fee.

It is also worth pointing out that government procurement is not
handled by Congress but rather by the departments or agencies, and
that bids are called for, allowing interested suppliers to bid. 
Whether anyone is looking at the people in the decision process for
the bids is a good question.  These two sites make it clear that
government employees would be "inside traders" if they utilize
non-public information from their work:

http://www.sec.gov/answers/insider.htm

http://management.itmanagersjournal.com/article.pl?sid=04/06/09/0334214&tid=84

Section 7 C of this Code of Conduct for Public Officials is pertinent:

http://www.immigration.gov.ph/code_of_conduct.php

Does this also apply to Congressmen and women and Senators?

No, they are not exempted, and yes, they could be prosecuted.  The
insider trading rules prohibit trading on material, non-public
information, regardless of the identity of the trader.

I've seen academic studies showing that members of Congress outperform
relevant benchmarks by 1-3%. Over a few terms, this compounds to be a
significant difference. (Just for fun, I did a few tests in Excel...
assuming the public gets 4% and Congress gets 6%, over an 8-year term,
Congress ends up with 16% more money.