I upgraded to WordPress 2.7 a few days ago and I've noticed that the Preview button seems to work only part of the time. After further testing, I've discovered that the Preview button DOES work, but only after the post is published.
If I click "Preview" prior to the post being published, it sends me to:
However, that does nothing except show me the front page.
I have tested this with the default theme (WordPress Default) and it behaves in the same way.