I think user-level automatic software updating would be most practical for sites where significant customizations only occur in the package/package-key/www and siteroot/www sections, mainly with the adp pages. Otherwise there are too many factors that need to be considered before an update, so that cvs / arch or whatever becomes the most practical tool.
I am considering writing a "global" etp (G-ETP) package that edits pages by first locating the adp file from within the siteroot/www tree, such as /subsite1/foo.adp. If no file exists, then the editor copies the files (necessary to make the page work) from the package-key/www/foo.adp associated with the url, and places them at siteroot/www/subsite1/foo.* for editing in a large textarea (like ETP).
An editor like this would be really convenient for end-user-admins because general templating can be modified to specific instances. A side benefit of using the siteroot/www tree is that APM updates will not affect the pages. The G-ETP could diff changes on request, validate etc.
Modifying sites this way would help developers to know when to put business-logic in package-key/tcl or /package-key/lib while retaining the features of templating for end-user-admins in the package-key/www area.
This is sort of how mac.com handles template editing (with the addition of webdav).
The main problem I see is that there is no current provision for the APM to "watch" or "load changed" xql files in the siteroot/www tree. I have added a feature request in the bugtracker.
