I need a brief overview of the how our understanding of mental
illnesses has changed from the early 20th century to present. 
Specifically how psychiatry and psychology have dictated how the
mentally ill should be treated, as well as how culture plays into
this, if convenient.  I'm not looking for a terrifically detailed
explanation, but lots of resources would be great so I can continue
this line of inquiry myself.  Thanks.

Hello, Mary Sue.

I've gathered thirty online resources for you, and have briefly
annotated each link.

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This is an excellent essay which includes a timeline highlighting
important 20th Century events and discoveries related to mental
illness and its treatment:

Mental Health World: 20th Century History of the Treatment of Mental
Illness: A Review
http://intotem.buffnet.net/mhw/29ap.html

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Several good histories and timelines:

A TimeLine of Psychological Ideas from 1900 to 1949 
http://www.geocities.com/Athens/Delphi/6061/en_linha2.htm

A TimeLine of Psychological Ideas from 1950 up to Our Days 
http://www.geocities.com/Athens/Delphi/6061/en_linha3.htm

Middlesex University: Mental Health History Timeline
http://www.mdx.ac.uk/www/study/mhhtim.htm#1900

Discovery Health Channel: A Brief History of the Treatment of Mental
Illness
http://health.discovery.com/convergence/killorcure/madness/madness.html

University of Calgary: History of the Treatment of Mental Illness
http://www.psych.ucalgary.ca/CourseNotes/f02/PSYC205PV/15TreatmentDisor.html

ColoradoPsycho: History and Perceptions of Mental Illness (A Timeline)
http://www.coloradopsycho.com/timeline.htm

Brock University: Timetable of Significant Events in Psychology 1840
to 1935
http://spartan.ac.brocku.ca/%7Elward/mead/time/TIME_PSY.HTML

Mind Publications: Notes on the History of Mental Health Care
http://www.mind.org.uk/information/factsheets/N/notes/notes_on_the_history_of_mental_health_care.asp

University of New Brunswick: HistPsyc Headlines Pages
http://www.unbf.ca/psychology/likely/headlines/index.htm

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This timeline of neuroscience contains many references that may be of
use to you (scroll down the page to get to the 20th Century material):

University of Washington: Milestones in Neuroscience Research
http://faculty.washington.edu/chudler/hist.html

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Here you'll find a history of schizophrenia and its treatment:

eMental-Health: History of Schizophrenia
http://www.emental-health.com/schiz_history.htm

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Histories of the use of electroshock and lobotomy in treating mental
illness:

23NLPeople: A Brief History of Electroshock (Electroconvulsive
Therapy)
http://www.23nlpeople.com/electroshock_history.htm

23NLPeople: Brief History of Lobotomy
http://www.23nlpeople.com/lobotomy.htm

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A history of the theories and treatment of depression:

Johns Hopkins Health After 50: A Brief History of Depression
http://www.hopkinsafter50.com/html/silos/depression/haARTICLE_history.php

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A look at the history of psychopharmacology in the management of
depression:

Social Audit Homepage: What Passes for Progress
http://www.socialaudit.org.uk/390what.htm

Social Audit Homepage: Treatment of Depression: The First 30 years
http://www.socialaudit.org.uk/23treatm.htm#2.3%20Treatment

Social Audit Homepage: Defeating Depression
http://www.socialaudit.org.uk/212DEFEA.htm#2.12%20Defeating

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A history of the self-help and advocacy movement in mental health:

National Mental Health Consumers' Self-Help Clearinghouse: History of
the Mental Health Self-Help and Advocacy Movement
http://www.mhselfhelp.org/pubs/history.html

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Here is a look at the management of mental health services from the
government's standpoint:

Office of the Surgeon General: Overview of Mental Health Services
http://www.surgeongeneral.gov/Library/MentalHealth/chapter2/sec7.html

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This page enables you to search the American Psychological Database by
keyword or by date:

Central Washington University: The American Psychological Association
Historical Database
http://www.cwu.edu/~warren/today.html

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An excellent collection of documents and links:

York University: History & Philosophy of Psychology Web Resources 
http://www.psych.yorku.ca/orgs/archcoll.htm

York University: Classics in the History of Psychiatry
http://psychclassics.yorku.ca/topic.htm

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Here's a well-written history of various theories of psychology, plus
a timeline and a map (see "Part Four:  The 1900's" at the bottom of
the page.)

Shippensburg University: The History of Psychology
http://www.ship.edu/~cgboeree/historyofpsych.html

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Some resources regarding culture-based attitudes toward mental
illness:

Western Washington University: The Mutual Embeddedness of Culture and
Mental Illness
http://www.ac.wwu.edu/~culture/Sam_Moreira.htm

Western Washington University: Online Readings in Psychology and
Culture
http://www.ac.wwu.edu/~culture/contents_complete.htm

Serendip: Mental Health, Cultural / Sociological Considerations
http://serendip.brynmawr.edu/sci_cult/mentalhealth/sociocultural.html

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Here are a couple of brief, but informative articles about changes in
attitudes about mental illness in relation to the workplace:

Reintegration: A Historical Perspective on the Treatment of Mental
Illness
http://www.reintegration.com/CONSUMERS/Employment/employment_workplace_Historical.htm

Reintegration: The Last Surviving Stigma
http://www.reintegration.com/CONSUMERS/Employment/employment_workplace_Historical.htm

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A list of books that may contain relevant source material:

Amazon.com: Listmania! History of Psychiatry by Hans Pols, Historian 
http://www.amazon.com/exec/obidos/tg/listmania/list-browse/-/3HIA3JWQOM96B/ref%3Dcm%5Flm%5Fdp%5Fl%5F1/104-8513768-8823128

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Search terms used:

"history of psychiatry" + "20th century"
"history of psychology" + "20th century"
"mental health" + "history" + "20th century"
"treatment of mental illness" + "20th century"
"psychological theories" + "20th century"
"attitudes toward mental illness"
"culture and mental illness"
"sociology of mental illness"
"history of psychopharmacology"

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I hope these links will be helpful. If anything's unclear, if a link
does not function, or if there is a specific topic upon which you need
more detailed info, please request clarification; I'll be glad to
offer further assistance before you rate my answer.

Best regards,
pinkfreud