I've got an XFX HD 6870 card that is supposed to support 4 monitors natively, in fact, that's why I bought it! I want to run 3x1 eyefinity, with 1 extended desktop. I have 3 Dell U2312HM monitors and an Acer monitor for the fourth (just to test with). I am hooked to the two Display Ports with 2 of the dell's and to one DVI with the 3rd dell. The Acer is hooked to the HDMI output.
I have Catalyst 12.4 installed and it shows the 3 dell's in 3x1 eyefinity, and when I go to "detect displays" it shows the acer there as a 4th monitor, but it says it's "disabled". If won't let me assign a position to it until I disable one of the 3 dell monitors.