I had expected that most XML Editor Vendors would integrate the DITA Open Toolkit functionality, since it provides so much capability already developed in an open-source community.

In a surprise, giant  Adobe is one of the earliest to do so. 

XMetaL was the leader in DITA OT integration. Arbortext Editor, Syntext Serna and others already could work with DITA because they had developed their own code.

Adobe's presentation on their DITA Application Pack DITA+FM is available as a (Macromedia) Breeze webinar. You can access it here:
http://www.adobe.com/cfusion/event/index.cfm?event=list&loc=en_us&type=o...

DITA+FM will be available for free download for FrameMaker 7.2 owners. A beta version is expected in August.