I'm not sure what the templating system has to do with XHTML? Template engines should output whatever is asked of them. One failure of acs-templating is adding whitespace in place of removed tags, but otherwise can't it output XHTML right now? Maybe you mean that templates need to look like valid XHTML?
How do html editors fit into OpenACS development? Will future work require their use?
The thing I don't understand about XHTML is how it fits into the needs of a site which allows users to provide dynamic content. Users would never be able to put in anything which wasn't perfect. That means a very smart input filter, and I can't see how the filter would be able to distinguish errors from intentional user input.
But maybe there is a good reason for using XHTML. What is it?
Are there any pages on OpenACS which could be delivered as application/xhtml+xml? If not, are there any pages which are close? Has anyone tried to convert them to see what they look like?
I still think the fastest way to explore this is to find another site/toolkit which takes dynamic input and see how they handle the issues.