I have been using my MacBook Core Duo (graphics card GMA 950) connected to a projector in dual display mode for over a year. After upgrading from 10.5 to 10.5.1, the macbook can function in mirror mode, but the projector does not receive a signal for dual display unless I restart the computer. The problem is consistent even though the projector has not changed.
Normally I plug in and the laptop screen changes to blue, then changes to dual display mode and sends a signal to the projector. Now, when I connect to the projector, the projector signal is black. I can switch to mirror mode using either F7 or via the Display preferences but there is no way to obtain dual display mode. Once I reboot, the problem is solved. This has been going on daily, each time with the same solution.
MacBook Core Duo 2 GHz, Mac OS X (10.5.1), GMA 950 graphics chip