Downloaded the ftplugin from here:
http://tritarget.com/cgi-bin/viewcvs.cgi/xmledit/ftplugin/xml.vim (pointed to by here: http://www.vim.org/scripts/script.php?script_id=301)
copied the file to ~/.vim/ftplugin/xml.vim
at this point XML completion works (.xml files) but html does not.
once the plugin was triggered and installed witha deleteme.xml file, I followed the instruction at did this:
I copy pasted the html.vim file in the help to a new file:
and then loaded my rhtml (or any html file that autocmds to filetype html) and the plugin started working