Moving to Ubuntu Single Sign On
That pretty much sums it up.
I've tried. Failed. Did I miss it? Feel free to call me out if I did. I'm OK with that.
Stack Exchange supports OpenID login, so you can add this identity to your account:
my logins link near the top.
add more logins...
https://login.ubuntu.com into the field for your OpenID (and clear the Launchpad username field if it contains anything).
Log In, and you should be presented with the same pages you see when logging in to the Ubuntu One site, for instance.
Once this is complete, you should be able to log in to your account on this site by using
https://login.ubuntu.com as the OpenID URL.
I think it is not among the list of recommended ways of signing into Askubuntu.com. The ones I know which are listed are, the Askubuntu, Stackoverflow, social networks and mail providers logins.
However, you are free to contact them through the 'Contact Us' or check the FAQ if your question is among answered ones.
AskUbuntu is a community based site developed and maintained by volunteers. It has emerged from Area 51 ( http://area51.stackexchange.com/ ) and is not (and can not be) linked directly with any enterprise (see: http://area51.stackexchange.com/faq ). Ubuntu One is a service from Canonical, the enterprise behind Ubuntu.