The fix should be in WP 2.1.2. Can those of you who are still seeing this confirm that you're running with WP 2.1.2? I confirmed against my own 2.1.2 test install that the bug seems to be fixed.
If you're on the WP 2.0.x releases still, it looks like it might not be fixed until 2.0.10 is released.
At least ... the right trackback address gets into the Wordpress field (instead of 'array'). The current problem seems to be that Wordpress doesn't submit the trackback ping unless you re-edit the field in Wordpress. Odd, that. But it doesn't seem like a MarsEdit problem, at least.
Glad to hear. Hopefully they can fix that ping problem on the WP end. I've found the WP team to be very friendly, and you might want to report a ticket at their bug reporting system: