I honestly can't remember what I used to login to Ubuntu Forums, but I am quite sure it was OpenID. Should I be concerned?
Since OpenID login is always processed on the issuer side (for example, if you use OpenID you got from Launchpad, then Forums will contact Launchpad and it is Launchpad who processes your authentication), the Forums do not keep your OpenID password (they don't need it at all).
Therefore, all the attackers can get is your OpenID login, but not the password, since it's not actually there.
Also, just for completeness, according to this official announcement, only the Forums are affected, no other services are.
