What's the name of our site?
AskUbuntu seems to be more popular currently, but that could just be that it gained some initial momentum somewhere. Our FAQ, however, says:
AskUbuntu is a Q&A site...
The name answer says Ask Ubuntu, which is also how the site is identified when logging in via a Launchpad OpenID (and maybe others? haven't checked). Stack Exchange convention is to use a space for the site name, and site names generally look better that way.
Personally, I'd rather see Ask Ubuntu, but it's not a big deal and I've been going with what has seemed the more popular choice. On the other hand, there are still many users who use StackOverflow rather than Stack Overflow, and if the currently most popular SE site gets that, certainly we will too.
On the gripping hand, it is important to have a consistent face, even if which option is used doesn't matter.