I have a MacPro with the ATI Radeon 5770 HD and I can't for the life of me get my 27" iMac to go into Target Display Mode. I've connected the ATI card to the iMac using the Belkin MDP to MDP and have both CPUs on while hitting Command + F2 (on the keyboard attached to the iMac) to no avail.
Both computers are fully update and running Snow leopard.
Apple doc:Summary
With Target Display mode, you can use your 27-inch iMac with Mac OS X as an external display. Connect any computer or other device with a Mini DisplayPort to your 27-inch iMac using a Mini DisplayPort to Mini DisplayPort cable or using a converter that converts other electrical, video, and audio protocols from another source device to Mini DisplayPort compliant signals. (Cables and converters available separately.)
Products Affected
iMac (27-inch, Mid 2010), iMac (27-inch, Late 2009)
To use a 27-inch iMac in Target Display mode with another computer as the source:
Make sure both computers are turned on and awake.
Connect a male-to-male Mini DisplayPort cable to the Mini DisplayPort on each computer. The 27-inch iMac will enter Target Display mode and display content from the source computer.
Note: If you are connecting two 27-inch iMacs, connect a Mini DisplayPort cable to each computer and press Command + F2 on the 27-inch iMac keyboard that you will use as an external display.
To leave Target Display mode, press Command + F2 on the keyboard of the 27-inch iMac that is in Target Display mode. To return to Target Display mode, press Command + F2 again.
Points to keep in mind:
Applications running on the 27-inch iMac computer remain open and running while it is in Target Display mode.
Use the keyboard of the 27-inch iMac to adjust display brightness and sound volume and to control media playback of applications running on the 27-inch iMac in Target Display mode. Other keyboard and mouse input is disabled on the 27-inch iMac while it is in Target Display mode.
The 27-inch iMac works like any other external display while it is in Target Display mode, except that you cannot access its built-in iSight or USB and FireWire ports. To change display settings, open System Preferences on the external source computer and choose Display from the View menu.
Mac OS X on the 27-inch iMac ignores some sleep requests while it is in Target Display mode, but forced sleep, restart, and shutdown commands will still work. If the external source goes into idle display sleep, the 27-inch iMac in Target Display mode will go dark until activity resumes on the external source.
If you shut down, sleep, or detach the external source while In Target Display mode, the 27-inch iMac will leave Target Display mode.
The Mini DisplayPort in the 27-inch iMac can receive only DisplayPort compliant video and audio signals. Converters not made by Apple may provide options to convert other electrical, video, and audio protocols to Mini DisplayPort compliant signals.
Some older Apple keyboards and keyboards not made by Apple may not allow the Command + F2 key combination to toggle display modes. You should use an aluminum wired or wireless Apple keyboard to toggle Target Display mode on and off.
Additional Information
Note: Target Display mode requires a Mini DisplayPort to Mini DisplayPort cable, such as the Belkin Mini DisplayPort to Mini DisplayPort cable, which is sold separately at the Apple Store as part number TX706VC/A.
I have about the same setup except for the 4870 which I may soon upgrade to the 5870. Target Display Mode works perfectly on my iMac. Sounds like you have everything right. The Belkin cable is the only one that works consistently. Others have proven problematic. Fully boot the iMac first, then the Pro. To troubleshoot, try swapping ports on the 5770, try a different (but still Belkin) cable. Once both computers are fully booted and awake, Command plus F2 is all it should take.
