After upgrading to wordpress 2.0, I found the WP2.0 is failed to send trackback.
This issue is also confirmed by ijliao, priv, and Ah Knight’s Blog.
There has been a workaround in Ah Knight’s Blog for this issue.But even you clean up the “to_ping” field, you may encounter this issue again later. Instead of clean up the “to_ping” field times by times, I try to dig the bug out from the source.