You mean your n900 rebooted after you install extkbd by itself? Or you restart it manually after installing extkbd and get caught in reboot loop? Will you explain what you actually did. Anyway, extras-devel is the first repository things fall into after passing through autobuilder. The software there is most probably not ready for end users, read here http://wiki.maemo.org/Extras-devel and here http://wiki.maemo.org/Extras-testing