Don Day just announced the release of the DITA Open ToolKit User Guide, which you can download from the Open Source website as a PDF or as HTML. Naturally, it was authored using DITA. My tiny contribution to some of the troubleshooting information pales in comparison to the amount of work that the team of Anna van Raaphorst and Dick Johnson did to get all the content into topics and tested thoroughly. It supports up to the most recent release of the ToolKit which is 1.2.2.
Don has a post on the dita-users Yahoo Group that talks more specifically about the efforts, other complementary documentation (like the DITA User Guide from the folks at Comtech and the version 1.0 language specification), and a call to continue to contribute and refine the content.
Great going, open source community! I expect to use the dickens out of this Guide and will contribute what I can as we continue with our DITA implementation work.