I'd like to confirm that the displayport-to-VGA adapter does work to provide a 3 monitor Eyefinity display.
Apparently it is active without the need for the additional USB power.
I have a 22" widescreen, flanked by two square 19" (pic taken with wide-angle lens, makes the 19" monitors look widescreen, but they're not).
The resolution is 3840x1024.
When I first hooked it up, I connected the displayport/VGA adapter to one of the 19" monitors - did not work. When hooked to the 22" monitor, it does.
Therefore, connect it to your monitor with the highest native resolution, and you're good to go! (thanks, whismerhill)
I bought the adapter from Directron. I live close to Houston, so walked in and picked it up. I was skeptical, but for less than $23, I gave it a shot.
Hope this helps.
This is the adapter:
This is where I got it:
http://www.directron.com/apdpvga.html
Here's my setup: