Where would I ask a question about a web-based content management system (e.g. WordPress)?
You should look at the Beta Wordpress Stackexchange after exploring the native Wordpress Support Forum (the Wordpress site has several searchable mailing lists, and you can ask questions / participate in the lists too).
For general CMS stuff, maybe the Beta Webmasters Stackexchange.
Additionally Stack Overflow does have a wordpress tag and several other Wordpress related tags.
Also in the proposal stage:
At some point, in the distant future, I'm sure the overlap between all of these sites will get worked out. Maybe.