I'm interested in finding out any benchmark data on the value of
information that major companies lose when employees leave due to 1)
Retirement or 2) All other reasons.  I'm particularly interested in
the effect of employee departure in an engineering/product development
environment.  The information must be attributable to some relatively
reputable source

I'm not an official google researcher, but I do work in the HR field
and can provide you with access to some very good benchmark research
articles. The following articles will most likely provide you with the
information you need. Too much information to list here, so I'm just
providing the links. Hope this helps,

Employee Turnover ? A Critical Human Resource Benchmark.
http://www.epf.org/research/newsletters/2002/hb20021203.pdf

Center for Community Economic Development
How Much Does Your Employee Turnover Cost ? Interactive Web Calculator.
http://www.uwex.edu/ces/cced/publicat/turn.html#calc

Compute Annual Turnover Costs 
Three options ? Management, Production, and Sales
http://www.employeeselect.com/computeTurnover.htm

Cost of Turnover Calculator - Interactive Web Calculator
http://go.sibson.com/row/cot/

Turnover Cost Calculator
http://www.insightpulse.com/employee_turnover_costs.cfm

Article - Calculating the High Cost of Employee Turnover
http://www.ilogos.com/en/expertviews/articles/strategic/20031007_YL.html

Article - Human Resources and R.O.I.
http://www.stakeholderpower.com/story.cfm?article_id=154

Article - Employee Life Cycle
http://www.stakeholderpower.com/story.cfm?article_id=219

Article - A new survey from PricewaterhouseCoopers LLP finds that
human resource leaders believe that organizations that align their HR
strategies with their business strategies are more profitable.
http://www.stakeholderpower.com/story.cfm?article_id=421

Study - 2001 National Employee Benchmark Study
http://www.askemployees.com/why/docs/National_Employee_Study.PDF

Article - Employee Turnover? The Costs are Staggering
http://www.itworld.com/Career/1993/ITW2491/

General Articles
http://www.shrm.org/
http://www.hr.com

Further to my previous comment post...here is some very specific
articles with regards to "information loss" related to employee
turnover and departure.

http://www.wrdi-institute.com/content/11_FAQs/11_C_Page_006.htm#4.%20What%20are%20the%20Effects%20of%20Turnover
However, direct replacement costs comprise only a small portion of the
total cost of turnover. Indirect or less visible costs (which are not
immediately apparent in an organisation's financial reporting)
include:
-lost customers or damaged customer relations; 
-additional marketing and sales expenditures needed to win replacement customers; 
-loss of intellectual capital; 
-lost opportunities (particularly in the development of new
technologies and products);
-a decrease in the "bench strength" required for effective succession planning; 
-advantage to competitors from their knowledge acquisition of their
new recruit's ex-employer;
-decreased morale and increased stress among remaining employees; and 
-the negative affect on an organisation's reputation.

Research Organizations Discover their Intellectual Capital
http://systemforschung.arcs.ac.at/publikationen/17.pdf

Protecting Trade Secrets and Other Intellectual Property in Employment Agreements
http://www.bromsun.com/pub/professional/lcb08.html

Safeguard Against A Potential Brain Drain
http://www.internetwk.com/change/change061499-2.htm

Are Employers Too Casual When It Comes to Intellectual Property Rights?
http://www.tht.com/Pubs/SearchMatchPub.asp?ArticleID=499

The Intellectual Property Audit
http://www.fenwick.com/docstore/Publications/IP/IP_Articles/IP_Audit.pdf

Intellectual Property Audits ? An Investment in Security
http://www.hyndco.com.au/IPAudit.htm

Trends In Corporate Knowledge Asset Protection - 
http://www.tlainc.com/articl47.htm

Hope this helps... educate-ga

Educate-ga:
Thanks for the helpful links.  I'm still trying to find something that
will allow me to calculate a real dollar amount for the loss of
knowledge.  However, the links you've provided do at least help to
clarify that the amount is very significant.
Thanks again.