I tried to trackback an entry on my wordpress powered blog, from another weblog of mine (which is powered by Movable Type), but it produced the following error (from my MT Activity Log):
Ping 'http://www.frish.nl/b2trackback.php/1' failed: HTTP error: 500 Internal Server Error
So I chmodded b2trackback.php to 777 and tried it again, still the same error.
Then I figured, perhaps is has something to do with the way, the URI is built up (mydomain.com/b2trackback.php/ENTRYID), so I tried to trackback mydomain.com/b2trackback.php?p=ENTRYID
This didn't produce an error, so I checked the entry-page I tried to trackback, but nothing showed up in the comments (and I checked my database, but they're not there).