I got this bug me too on a Toshiba U2OO laptop with 945GMA.
The screen blanks and sometimes reappears. If not I can press again the fn key and it reappears.

Output (with comments)
$ tail -f /var/log/Xorg.0.log
        No refresh rate overrides will be attempted.
(II) I810(0): Display plane A is disabled and connected to Pipe A.
(II) I810(0): Display plane B is enabled and connected to Pipe B.
(II) I810(0): Enabling plane B.
(II) I810(0): Display plane A is now disabled and connected to Pipe A.
(II) I810(0): Display plane B is now enabled and connected to Pipe B.
(II) I810(0): PIPEACONF is 0x00000000
(II) I810(0): PIPEBCONF is 0x80000000
(II) I810(0): Mode bandwidth is 61 Mpixel/s
(II) I810(0): maxBandwidth is 1152 Mbyte/s, pipe bandwidths are 268 Mbyte/s, 0 Mbyte/s
# Press fn key
(II) PM Event received: Capability Changed
I830PMEvent: Capability change
(II) I810(0): Next ACPI _DGS [0] 0x80000100
(II) I810(0): ACPI Toggle devices 0x1
(II) I810(0): ACPI Toggle to 0x1
(II) I810(0): Hotkey switch to 0x800.
(II) I810(0): Attachable device 0x80000100.
(II) I810(0): Attachable device 0x80000410.
(II) I810(0): Requested display devices 0x800.
(II) I810(0): Toggle (1) 0x1
(II) I810(0): Detected duplicate devices. Toggling (0x1)
(II) I810(0): Detected display change operation (0x900, 0x800, 0x1).
(II) I810(0): Clearing Clone mode
(II) I810(0): Primary pipe is now A.
(--) I810(0): A non-CRT device is attached to pipe A.
        No refresh rate overrides will be attempted.
(II) I810(0): Display plane A is enabled and connected to Pipe A.
(II) I810(0): Display plane B is disabled and connected to Pipe B.
(II) I810(0): Enabling plane A.
(II) I810(0): Display plane A is now enabled and connected to Pipe A.
(II) I810(0): Display plane B is now disabled and connected to Pipe B.
(II) I810(0): PIPEACONF is 0x80000000
(II) I810(0): PIPEBCONF is 0x00000000
(II) I810(0): Mode bandwidth is 61 Mpixel/s
(II) I810(0): maxBandwidth is 1152 Mbyte/s, pipe bandwidths are 268 Mbyte/s, 0 Mbyte/s
(WW) I810(0): Changing XVideo pipe (1 to 0).
# Blank. Press fn key
(II) PM Event received: Capability Changed
I830PMEvent: Capability change
(II) I810(0): Next ACPI _DGS [0] 0x80000100
(II) I810(0): Next ACPI _DGS [1] 0x80000410
(II) I810(0): ACPI Toggle devices 0x801
(II) I810(0): ACPI Toggle to 0x801
(II) I810(0): Hotkey switch to 0x1.
(II) I810(0): Attachable device 0x80000100.
(II) I810(0): Attachable device 0x80000410.
(II) I810(0): Detected last devices (2).
(II) I810(0): Requested display devices 0x1.
(II) I810(0): Detected display change operation (0x800, 0x1, 0x900).
(II) I810(0): Clearing Clone mode
(II) I810(0): Primary pipe is now B.
(--) I810(0): A non-CRT device is attached to pipe B.
        No refresh rate overrides will be attempted.
(II) I810(0): Display plane A is disabled and connected to Pipe A.
(II) I810(0): Display plane B is enabled and connected to Pipe B.
(II) I810(0): Enabling plane B.
(II) I810(0): Display plane A is now disabled and connected to Pipe A.
(II) I810(0): Display plane B is now enabled and connected to Pipe B.
(II) I810(0): PIPEACONF is 0x00000000
(II) I810(0): PIPEBCONF is 0x80000000
(II) I810(0): Mode bandwidth is 61 Mpixel/s
(II) I810(0): maxBandwidth is 1152 Mbyte/s, pipe bandwidths are 268 Mbyte/s, 0 Mbyte/s
(WW) I810(0): Changing XVideo pipe (0 to 1).
# Display OK