DITA in a nutshell:

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 1.1 architectural spec has a lot of decent intro material, and is up to date:
* http://dita.xml.org/standard

A very short and high-level intro to the architecture:
* http://dita.xml.org/dita10

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


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.