tedstryk and Gsnorgathon posted a message on the MER forum regarding satellite mutual events as seen from Cassini. Mutual events are occurances where one satellite gets between another satellite and the sun or between another satellite an the observer. The former case is impossible in the current orbital position, but now that Cassini is much closer to the Saturn's equatorial plane, the latter case is. The post, linked above, mentions one such mutual event with Tethys and Dione and is best seen animated, as Gsnorgathon describes.
Update: Turns out that the moon in the background is not Tethys, but is instead Rhea. Also, MizarKey at the forum thread linked in this post's title has created an animated gif showing the mutual event.
Update2: Thanks to the Solar System Simulator
for confirming the background moon as Rhea
(compare with the Cassini view