I'm just getting ready to update to 2.0 and am currently using 1.5.2.
I've had problems sending pingbacks/trackbacks: sometimes they go through and sometimes they don't. (I receive them fine, though.)
I just noticed it only happens with the posts I set to publish in the future. Any trackbacks/pingbacks in pre-published posts are handled differently, depending on the blogging client.
For example: this weekend I wrote up a post Saturday night to publish Sunday morning. I edited the time stamp, hit the Publish button, and got a list of URLs that had "already been pinged" even though the post wasn't published yet.
A Blogspot user contacted me soon after to tell me she'd received a trackback from my blog but she couldn't couldn't find the link on my blog. I did send her a trackback but it wasn't supposed to appear until the post published Sunday morning.
I checked the people I pinged/trackbacked: Blogspot people got pinged when I pre-published, but WordPress people never got pinged.
The first time this happened, MacManX took a look at his logs to see what happened (not knowing I had pinged him in a pre-published post) and he said it appeared as if:
1. I had published the post and sent a pingback.
2. I then deleted the post.
3. When his blog returned to my post to confirm the pingback, it found that the post no longer existed, so it did not post the pingback.
4. When the post finally did publish, either my blog declined to send a pingback because it had already sent a pingback to /of-handles-and-real-names/, or receiving blogs declined to accept my pingback since it had already received a false pingback.
Again, this is what he theorized as what might have happened, not knowing this was a pre-published post.
I include this info to show you my blog is successfully pinging other WordPress blogs in pre-publishing, but WordPress blogs aren't accepting pre-published pings because the posts don't exist yet.
This might clear up in 2.0, but I wanted to mention it in case 1.5.2 people are having problems pre-publishing posts with pingbacks/trackbacks.