I was unable to send trackbacks and pingbacks after I tested with WordPress 1.5.1.2. I found the solution yesterday.

I had too many XMLRPC Ping servers on my list and the php process was timing out while still pinging to them and before it could execute the trackback/pingbacks associated with the post.

The obvious solution is to reduce the list of servers you ping. I adopted a different approach. I increased the timeout to 5 minutes instead.

BTW: Check your pingback/trackback sending ability at Pingomation.

Let me know if it addresses your problem.