To me, this is obviously a subset of superuser.com. There is a small debate about this on area51. Quoting:
I think this is better than SuperUser because it's focused on Ubuntu, allowing those not interested in Ubuntu to ignore this content entirely rather than spamming it. I think it's better than the proposal you link to for the same reason. Ubuntu forums is a mess, and StackEngine is an extremely effective and well-designed platform for creating a knowledge base. The forums have served us well but forum threads quickly become unwieldy and they do not age well. – David Siegel Jun 20 at 17:40
It seems to me that a technical solution could solve this.
You could have a Stack Exchange site that operates against the same database as superuser.com, but forces every question to be tagged "ubuntu" and only ever displays questions tagged "ubuntu". This would make everybody happy, since if you only care about Ubuntu and no other operating systems that are discussed on superuser.com, you can easily filter everything else without any configuration hassles. If you care about the community on superuser.com, you get a larger number of people who are providing quality questions and answers - easily filtered by their subject of choice.
The same could be said about Stack Overflow; you could create "sub-sites" for different programming languages, etc.