Is there any way to take an image of a "cloud" server (the server in question is on the Rackspace Cloud/Mosso) and store it so that the server instance can be shut down/destroyed, and if I decide I want the server back, I can just restore this backed up image of the filesystem onto a new server instance?
rsync (as root) the whole filesystem somewhere.
Yes, you can create custom images based on your cloud server that can be used to create new servers. Check the Rackspace Cloud Servers API documentation for more information about how to do that.
When a server instance is destroyed the backup goes with it (that's a different action from turning it off or rebooting it, BTW - this is "canceling/destroying" the image.
Shutting off the instance still incurs billing (for the basic instance) but avoids usage charges such as bandwidth. Tip: Resizing an image to a small size and shutting it off can mean paying less than $20/mo to maintain the image for on-demand restoration/cloning.
I've been advised by Rackspace that they plan to add this functionality at some point in the near future (end of 2009 or early 2010).
That functionality when implemented would allow you to do something along these lines:
(using the management GUI or the API)