Two template tags have been added:
If you have posts in multiple categories, you can use get_referrer_category() to retrieve the category object, based on the referrer url.
get_referrer_url() returns the full URL of the referrer.
Also, there are two extra arguments for previous_post_smart() and next_post_smart(): $in_same_cat and $excluded_categories.
This is how the function prototype looks like:
previous_post_smart(
$format = '« %link',
$title = '%title',
$fallback = true,
$in_same_cat = false,
$excluded_categories = ''
);