Last I heard, Mark has no intention of Open Sourcing the server side, because he doesn't believe it would be a good business decision.
I, personally, would love for the backend to be open sourced. It's nice to run your own cloud, but what happens when your house burns down, or you get robbed? An offsite backup is best, especially because you can depend on a business which has more resources than an ordinary home user to ensure the data is safe.
I refuse to pay for UbuntuOne partly because it is closed source, but mostly because of the name. You might find this silly, but that which is "Ubuntu" has been redefined to be open source, and open to the community only when the rights holders say so. [[COLOR="Blue"]_Names are Important_[/COLOR]]("http://www.chuckfrain.net/2009/05/18/ubuntu-one-thoughts"). If they open sourced the backend OR changed the name, I would subscribe Today. Until then, I have open source options (with better privacy guarantees) from other places, such as spideroak.com.
I'd love to support Canonical financially, because I got Ubuntu for free and I really appreciate the efforts Canonical has gone to to give me such an outstanding product. I'm just waiting for them to offer me something I value which I also don't find offensive, before I will sign up.