Update: this may have been sorted in v2.5.1 - will look into it.
There’s a documented bug in Wordpress 2.5 where the previous_post_link and next_post_link functions stop excluding the categories they’ve been told to.
The bug is not scheduled to be sorted until v2.6 for some weird reason (it’s only a minor change) but in the meantime you can fix it yourself by editing a line in ~/wp-includes/link-template.php
Replace line 557
$posts_in_ex_cats_sql = " AND tt.term_id NOT IN (" . implode($excluded_categories, ',') . ')';with the following:
$posts_in_ex_cats_sql = " AND tt.taxonomy = 'category' AND tt.term_id NOT IN (" . implode($excluded_categories, ',') . ')';Just need to be careful of this file when upgrading.
Full credit to filosofo on the wordpress forum for this.
