DITA in a nutshell:

An easy tutorial at the level of "what you need to know to get started:"
* http://www.xmlmind.com/tutorials/DITA/index.html 

A high-level intro:
* http://www.oreillynet.com/xml/blog/2005/10/going_dita.html

A short case study/interview:
* http://thecontentwrangler.com/article/astoria_softwares_move_to_dita_an_...

The new DITA 1.2 Spec has a lot of decent intro material in the Architecture section:
* http://dita.xml.org/standard

* (direct) http://docs.oasis-open.org/dita/v1.2/os/spec/DITA1.2-spec.html 

A very short and high-level intro to the architecture:
* http://dita.xml.org/dita101

An hour-long CAMTASIA video intro adapted from a recording of a live workshop Michael gave on DITA a few years ago - this is the first part, which has a high-level overview of DITA's history and capabilities:

* http://www.ditausers.org/tutorials/basics/Priestley_Intro/

DITA and Eclipse:

* http://dita.xml.org/dita-and-eclipse-powerful-combination (first half is about TaskModeler, an Eclipse-based DITA map editor - second half is on integrating DITA publishing with Eclipse)

Introduction to the DITA Open Toolkit from Robert Anderson, its lead architect:

* http://metadita.org/dita2007.html

DITA Project Starter Kit - A set of sample topics, a map, and ant scripts to get started or experiment with DITA

*http://dita.xml.org/blog/dita-project-starter-kit

General resources:
http://dita.xml.org/
OASIS's community web site for DITA. Has blogs, a Wiki, lists of adopters, vendors, presentations etc.

http://xml.coverpages.org/dita.html
Robin Cover's list of DITA resources. Includes a lot of presentations, news, etc. in chronological order.

http://www.oasis-open.org/committees/tc_home.php?wg_abbrev=dita
Official OASIS DITA Technical Committee site

http://docs.oasis-open.org/dita/v1.1/OS/overview/overview.html
DITA 1.1 Specification overview - including links to architectural spec and language spec. The architectural spec in particular is designed to be generally readable, and although it does get very technical, it also includes a lot of introductory and explanatory material.

http://dita-ot.sourceforge.net
DITA Open Toolkit site - go through the intro PDF cited above before going here.

http://www.ditainfocenter.com
The architectural and language specs, plus the Open Toolkit User Guide in an online Help (Eclipse) format wioth index, search, and table of contents.

http://www.ditanews.com
Listing of 50 DITA Tools, dozens of presentations and tutorials, communities, conferences, mailing lists, and a monthly DITA Newsletter.

http://www.ditausers.org
The international community with personal online workspace folders, a web-based DITA Storm editor, and an online Open Toolkit. Author, build, and publish your first DITA Topics and DITA Maps without installiing anything.