Since upgrading to WP 3.0 the Post Navigation Widget continues to work on individual post pages as before.
But on the main index page the widget's behaviour has changed ...
Firstly it appears on the main page, where in 2.9.2 it was absent.
Secondly, on the index page alone, the post that is shown is always the same and is not current (it's around a year old).
So slightly broken rather than fully broken but for now I've had to disable the widget, which is a shame. Fingers crossed there'll be a 3.0 compatible update at some point.