Ironically, the recent post I wrote about upgrading painlessly to Wordpress 2.0 not working correctly (as the trackback link led to a 404 error), so thanks to her for letting me know. The link works now.
The odd thing is that not all permalinks are broken, only that post as far as I know. Several other threads on Wordpress’s support forums highlighted the problem, but mine was specifically a WP-generated 404 error (that is, the 404 was displayed within my blog, styled, etc.), which means WP can’t find the page within itself, not that the server can’t find it.
Anyhow, if anyone else has been suffering this pain the solution was a patch by Ryan Boren, one of the WP contributors. So I owe him a debt. You can find the patch and info on this support thread here.
UPDATE: Another part to this story seems to be that it was to do with the timestamp, rather than anything else. The trackback URL on Lorelle’s site from me had the timestamp of 01/02 (2nd of January for us Europeans) but my post timestamp was 01/01. No idea why that should be or how it happened. So I’m not sure if Ryan’s patch made any difference at all, but I’ll leave it in anyway. I restamped the time to be 02/02 and now everything should work (unless you linked to the old one, heh).