There was a link on a popular Facebook 'Ubuntu fan page' to the site today. Somebody commented to ask "Why can not use Launchpad username and password??".
Of course you can use your launchpad account - by entering your launchpad profile URL in the OpenID login – e.g. https://launchpad.net/~fred-bloggs
In fact we could use the Launchpad Single Sign On (aka Launchpad sign on) as used by most official Ubuntu webpages, because Launchpad Single Sign On is as I understand it, effectively an OpenID provider.
It would be wonderful if by the Ubuntu Stack Exchange official launch there was an obvious option on the login page to use this 'standard Ubuntu account'. Without this, we may alienate many potential users of the site.
Edit: I asked about this at Meta StackOverflow