Yahoo openID + OAuth の実装を確認したところ、この警告メッセージが表示されました
WARNING - PLEASE READ
Yahoo is strongly encouraging Yahoo! end-users not to authorize any new applications using OAuth until the recently discovered security issue with the OAuth protocol is resolved. Yahoo! is not aware of any exploits of this issue prior to April 23, 2009.
If you click the "continue at your own risk" button below, your private data may be exposed to unintended third parties.
Note: Unless you click "Continue at your own risk," your data has not been shared.
Note to Developers: Developers who are authorizing their own apps, for the purposes of testing, may proceed at their own risk. Yahoo! recommends that developers create test accounts with test data for testing their application.
どうやら、OAuth で何か怪しいことが起こっているようですが、Google がこの点に関して何らかの発表をしたかどうか、または Plaxo はわかりません!