quicklauncher installs <prefix>/share/xfce4/panel-plugins/quicklauncher.desktop with X-XFCE-Module=/home/martin/opt/xfce-svn-26054/lib/xfce4/panel-plugins/libquicklauncher.so in my case, which prevents xfce4-panel from loading this plugin. According to the documentation i found, it should contain: X-XFCE-Module=libquicklauncher.so X-XFCE-Module-Path=/home/martin/opt/xfce-svn-26054/lib/xfce4/panel-plugins with these entries in quicklauncher.desktop, it loads correctly.
Must be fixed in trunk, thanks for reporting.
Same problem is discussed in bug 5042.