How does one find and replace text in all open files with kate?
In Kate 3.8.5-
Go to Settings -> Configure Kate -> Plugins and enable there Search & Replace. Then use the "Search and Replace" button that appears at the bottom of the Kate main window to find the desired functionality.
The answer is simple. As of Kate 3.4.3 (present in KDE 4.4.3) you can not replace in multiple files at once but just in the one you're currently viewing calling "edit->replace" or with the CTRL+R shortcut.