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Earlier there was some XOOPS core diagram available (now its gone :\). I guess it was made by kazu with Dia.
Now U can find even better uml-software choise, even with php reverse-engineering
http://www.phppatterns.com/index.php/article/articleview/98/1/11/
http://www.sparxsystems.com.au/php.html
http://www.waterproof-software.com/uml/
http://www.microgold.com/version2/articles/article3/PHPReverse.html
I've seen a pdf recently with some UML, but I guess it was about User class and maybe core. And I wanted core, module, amd especially comments feature (as I want to do some comments search module)
DB relation diagrams would be also a pleasant thing, like drupal fellows have
http://lists.drupal.org/pipermail/drupal-support/attachments/20030530/7a6fd04a/cdm1-0001.png