Some time ago, Apple released a preview of this for Panther; back then itwas called "Spoken Interface". DO NOT INSTALL THIS UNDER ANY CIRCUMSTANCES; 10.3.6 introduced changes which conflict with the Spoken interface preview so badly that the machine will become unbootable, and later updates have the same problem. How do I know this? I used those previews, for a time. I'm not blind, but my work involves Web accessibility, and so I thought it would be a good idea to look into this.
As I understand it, Voiceover is not a screenreader in the traditional sense. Programs present their interfaces to it via an API, and it translates those interfaces into spoken text. As usual, Cocoa got it all for free but Apple dragged its feet on the Carbon side. Of course, the preview was released a year ago (at the last MWSF, I believe) and never updated, so things could have changed.
Voiceover was definitely not ready for prime time when it came to Web browsing. In particular, it had the annoying effect of speaking out all links twice, but giving no actual indication that they were links. If my memory serves, it only spoke the address of links, rather than the actual text; this is another problem. Of course, Gecko didn't work with it at all, but that's to be expected given that the accessibility APIs had not yet been released.
The good news: this was all a year ago. Many things have doubtless changed, and the bug I mentioned must have been fixed. However, it remains to be seen just how good Voiceover will be.