I posted this a few days ago to the dev-list and now I'm ready to send it on to a wider audiance...
Since I have this sadistic desire to serve up my pages as application/xhtml+xml, as that the way xhtml documents are "supposed" to be served up. I made a quick and dirty hack to "fix" Serendipity to yeild to my desires. Christmas day I coded up a proper fix.
I made a Smarty plugin-function that will do everything I need... from generationg the proper doctype... and even setting a varaiable to use in the html header for the matching content-type.
The only down side to my current patch, it doesn't have an upgrade option just yet...
You can check out the patch here... http://riscky.info:8080/serendipity/archives/7-Proper-XHTML-Solution.html
Let me know what you think.