我已经解决了.

事实证明这是Android 2.0中的一个错误.
如果您在 AccountManager 中添加一个帐户，则在 Android 2.0 平台下，您还必须为该帐户提供一个 SynAdapter.但是在Android 2.1 及更高版本下一切正常.

It turns out that it's a bug in Android 2.0.
If you add an account to the AccountManager, you must also provide a SynAdapter to the account, under the Android 2.0 platform. But things are all right under Android 2.1 and above.

这是一个已知问题，请参考:
http://code.google.com/p/android/issues/详细信息?id=5009
和
AccountManager 没有 SyncAdapter?

This is a known issue, please refer to:
http://code.google.com/p/android/issues/detail?id=5009
and
AccountManager without a SyncAdapter?