We attempted to confirm the release with Apple PR directly, but we haven't heard back. Since this isn't showing up on apple.com/pr, and as of yet it hasn't been released to Software Update, it's understandable that the PR Newswire release might be fishy to some. However, I used to work for PR Newswire, and I can say with 100% confidence that this release is legitimate; PR Newswire's releases come directly from the companies themselves.
We should see Safari 5 officially released sometime today.
Update: The press release has been pulled from PR Newswire. Based on my experience there, most likely this means the release was published prematurely. It will most likely go back up in the next few hours, around the time it was originally intended to be published. Meanwhile, the text of the press release is still viewable on Engadget.
Update 2: It's live! Here's the link.