I noticed today (and a google search confirmed it) that there's at least one thing in WordPress 2.7.1 that breaks in the Safari 4 beta. The floating URL entry box in the story editor goes dark all over and you have to refresh the whole page to get it back. I had to switch to FF today after installing the Safari beta, just to finish posting a story with urls (didn't feel like entering all that html).
Here's a blog post with a photo of the problem that I found via google:
