If you are looking for real-world technical communication examples of XML, be sure to check out The Rhetorical Nature of XML: Constructing Knowledge in Networked Environments, by J. D. Applen and Rudy McDaniel. The final chapter has several case studies, one of which I contributed from my work at FarPoint Technologies (before they were purchased by GrapeCity). The book’s web site is http://rhetoricalxml.com and the wiki where several of the case studies are readily available are at http://rhetoricalxml.wikispaces.com. I will let you read it and decide whether the authors are correct about their assessment of the rhetorical power of XML tagging.
There are more uses of XML than can be covered in one book, but my cooperation with the authors was an attempt to show that those efforts to use something as universal as XML do not have to be complicated or expensive. I hope that this book, which studies the use of XML at the intersection of technical communication and knowledge management introduces another generation of technical communication professionals to the flexibility and power of XML in the real world. My apologies to the authors for not blogging about their work more earlier; it came out as I was transitioning from one job to another.