So I have the HD 5870 on a 3.1 Octo. I have the DVI connected to A 27" Acer (works fine), and the MDP to a Samsung SyncMaster 226. Using the MDP to DVI adapter won't work on either port. I tried the regular MDP to DVI and the $99 Apple MDP to DUAL link DVI adapter. However, if i use the MDP to DVI adapter connected to a DVI (female) to DVI (male) adapter, it does !!! Can anyone explain this ? Also, I tried the same thing from my MacBook Pro connected to my Panasonic Plasma TV with a DVI to HDMI adapter having the same situation. What does the DVI to DVI do that solves the problem? Hopefully this will help others not being able to use a second and/or third monitor.