I've got 2x20" Dell 2007's @ 1600x1200 for my left and right screens (left has the DP>VGA), and a 24" Dell 2405 @ 1920x1200 for my centre screen. Of course with Eyefinity they will all run at 1600x1200 for a total of 4800x1200.
I did a bit more fiddling around, this time I was able to get all 3 displays running in Eyefinity on the desktop after unplugging/plugging the DP>VGA a few times. But as soon as I tried loading Dirt 2, all screens went black and my PC locked up.
At the moment I've got all 3 displays going with the normal desktop extend. I'll see if this lasts without dropping out, but for now Eyefinity is still out of reach. :(
Here's what I do, I boot up with the DP>VGA adapter attached, it shows the extended desktop with 1 primary 2 extended, Displayport one working fine. I go into Catalyst, set each one of the 3 to Duplicate instead of extend. At this point the DP connected one goes black. Unplug, and plug that one back in. Close Catalyst, Open Catalyst again, now set that one to Duplicate. That time it'll stick and you will have the same image on all 3 screens. Then click the down arrow on one and enable the Eyefinity mode.