Last night I changed the password for my Stack Exchange OpenID. As a security precaution, I would expect that all of my sessions be invalidated and to be asked to log in again on all sites where I use the OpenID - after all, if someone gets my password and logs in as me before I can change it, changing it is next to pointless.
When I came into work this morning, I opened up Server Fault and to my surprise, I was in fact still logged in. Doing a bit of checking, I am also still logged into all other Stack Exchange sites I was pre password change.
I don't know the inner workings of how OpenID works, so this may not even be possible, however if there is any way all existing sessions can be invalidated upon a password change then please consider implementing this (maybe as an option on the password change page, if that's possible too).