Suddenly couldn't login to Stack Overflow or it's "family" (Meta, ServerFault, SuperUser etc.) using my Blogger OpenID. (Cue: Major panic).
Mr. Google to the rescue - the answer is here in this article How to upgrade your Blogger OpenID to a decent one.
Note: I did not get the length error reported in some posts but the above sorted out my problem.
Tip: To ensure this doesn't happen again, get an alternate OpenID - I used Google. Just click on your user name at the top of the StackOverflow screen and then click "Change OpenID" (next to the "Edit" hyperlink). You then get an "openid" and a "alt openid" and you can swap the two around using the "swap" hyperlink.