Even though DreamScene is still technically a preview release, Microsoft has just posted a little Content Pack featuring 4 new animated backgrounds.
The four videos are:
A field of thistle, with a bee gathering pollen
The rushing water of a forest stream
A streetlamp reflected in a puddle, with softly falling rain
Orange wisps flowing in a computer-generated scene
"We're offering these video backgrounds only to users who have installed the Windows DreamScene Preview. However, the Content Pack will continue to be available when the final version of DreamScene is released, so don't worry about installing the preview just so you can obtain them. The preview version is an unsupported release; it is not appropriate for users who are uncomfortable with installing prerelease software. If you've installed the Windows DreamScene Preview, click Check for updates in Windows Update to see the Content Pack, which is offered as an Ultimate Extra."