Just as a note onto this:
I did a little investigation, and changing the User Agent in the Safari Develop menu can activate the flash uploader. It seems that if it detects a mac based browser the flash muliple file uploader won’t be inserted and it falls back to the original single file version.
Here’s a list of user agents that do/don’t render the flash uploader:
Safari 3.1 Mac – Single
Safari 3.1 Win – Flash
Safari 2.0.4 (Mac) – Single
Safari 1.* (Mac) – Single
IE 7 (Win) – Flash
IE 6 (Win) – Flash
Firefox 2.0.0.12 Mac – Single
Firefox 2.0.0.12 Win – Flash
Firefox 1.5.0.12 Mac – Single
Firefox 1.5.0.12 Win – Flash
Opera 9.25 Mac – Single
Opera 9.25 Win – Flash
If you leave the user agent set to automatic it defaults to Safari 3.1 Mac (as that’s what the browser is) and as such the flash uploader isn’t there.
BTW I have the latest Flash Player installed (9.0.115.0), and (now) have WordPress 2.5 GM installed on my site.