hi, just wanted to forward this bugreport from launchpad (https://bugs.launchpad.net/ubuntu/+source/xfce4-indicator-plugin/+bug/778998), which states that the indicator plugin doesn't support the xfce4.8 panel's "real/alpha transparency". when changing the alpha the rest of the panel background starts to fade, but the indicator plugin stays with the non-alpha background. same behavior when using a background image.
fixed in git