odd user.nickname() behavior with OpenID

[decitrig]

I just recently discovered that the output of user.nickname() can change suddenly: I was using it to generate XSRF tokens, and I discovered that right after authenticating with Yahoo nickname() was "http://me.yahoo.com....gibberish....", while very shortly thereafter (fast enough that the POST would get rejected for a bad XSRF token), it would turn into my usual yahoo.com email address.

The behavior *seemed* to be limited to Yahoo. Has anyone else experienced this? Am I expecting too much from the User object with federated login?