I was using my https://login.ubuntu.com/ account to connect to http://ubuntuforums.org which has been corrupted. I've used it also for other services u1, launchpad, etc. After the recent attack, should I consider all my accounts on these sites to be corrupted?
No. Your Ubuntu SSO account should be fine, as well as any accounts other than Ubuntu Forums on which you used it.
Ubuntu Single Sign On was not compromised in the attack on Ubuntu Forums. Fortunately, no Ubuntu services other than Ubuntu Forums were affected. This is according to the information page now up at http://ubuntuforums.org/announce.html, which should be reliable since that domain is no longer controlled by the crackers who perpetrated the attack, nor by the system they compromised. Provided that you never created a separate account on Ubuntu Forums, you should be fine.
However, if you did ever have an account on Ubuntu Forums, and you use the same password on Ubuntu SSO (or any other site, Ubuntu-related or not), then you should make sure to change your password and stop using that password.