Are you using an ACTIVE displayport? http://www.amazon.com/Accell-UltraAV-B087B-002B-DisplayPort-Dual-Link/dp/B002ISVI3U
To enable Eyefinity a PASSIVE adapter http://www.google.com/products?q=displayport+to+displayport+dongle&aq=f
will not work (although there are suggestions that it will under certain situations).
In my setup, a PASSIVE adapter did NOT work (I had the same problem as you) BUT an ACTIVE displayport adapter will work.
Note: Do not buy an Apple displayport. I heard there are problems.