Joel Spolsky just tweeted, "the official ubuntu q&a is live on stack exchange!" However, it's not "on stack exchange". It's at askubuntu.com. Weren't we just informed that all the sites are going to be sub domains of stackexchange.com? Why the exception for this site?
SE had an agreement with Canonical with regards to SE possibly replacing the current Ubuntu forums. As explained on Ubuntu MSE:
Due to Stack Exchange's prior agreement with Canonical Ltd., ubuntu.stackexchange.com will recieve your askubuntu.com domain name, as orginaly agreed.
In exceptional cases like this, where
We have a an agreement in place with an existing outside community
The outside community's goals (Ubuntu, Inc.) are aligned with ours (Stack Overflow Internet Services Inc.) -- namely "make the internet better"
The domain name is quite good (there is the possibility of redirecting from ask.ubuntu.com if this agreement works out)
Then custom domain names are possible.