I was experiencing this problem and I tracked it down.
First of all, from what I saw, it is INCORRECT to say that the delimiter should be:
" and "
Meaning with quotes. There should be no quotes, it should just be
and
That is, a space, the word ‘and’, and then another space.
Either way, this was NOT working for me. I do not know why, but PHP was not running the explode() function correctly on my string, which appeared to be delimited correctly.
I hacked the WordPress code on line 884 of /wp-includes/link-template.php and changed where it said:
array_map('intval', explode(' and ', $excluded_categories));
to be:
array_map('intval', explode('|', $excluded_categories));
And then in my own code for the next_post_link() function, I did it like this:
next_post_link('%link', 'Next Post »', FALSE, '7|3|4');
It used to say ‘7 and 3 and 4’. Now it says ‘7|3|4’. Got it? Mine’s working okay now.
