I mentioned before that you can't just use any old OS with UEC; you have to supply it with a specially-prepared operating system image. Canonical has a few, but if you want to create your own image, you can.
As you can guess, this isn't a trivial process. You need to provide a kernel, an optional ramdisk (for the system to boot to), and a virtual-machine image generated using the vmbuilder tool (). It's also possible to use the RightScale, a cloud management service that works with Amazon, RackSpace, and GoGrid as well as Eucalyptus-style clouds. Obviously you'll need a RightScale account to take advantage of this feature, but the basic single-account version of RightScale is free, and has enough of the feature set to give you a feel for what it's all about.
So what's next for Eucalyptus on Ubuntu? One possibility that presents itself is using Eucalyptus as an interface between multiple cloud architectures. The folks at Canonical have not planned anything like this, but the potential is there: Eucalyptus can, in theory, talk to any number of cloud architectures, and could serve as an intermediary between them -- a possible escape route for clouds that turn out to be a little too proprietary for their own good.
Another, more practical possibility is more in the realm of a feature request -- that the existing process for creating, packaging, uploading and booting system images could be automated that much more. Perhaps the various tools could be pulled together and commanded from a central console, so the whole thing could be done in an interactive, stepwise manner.
What Eucalyptus and UEC promise most immediately, though, is a way to take existing commodity hardware and make it elastic without sacrificing outwards expansion. What you create with UEC doesn't have to stay put, and that's a big portion of its appeal.