I did try those setting (and a lot more, some extremely conservatives) for Monitor0 but to no avail. Xorg.0.log correctly reports that it found a i830M chipset.
Also, in Xorg.0.log there is those 4 lines
(II) intel(0): Output VGA using monitor section Monitor0
(II) intel(0): I2C bus "CRTDDC_A" initialized.
(II) intel(0): Output TMDS has no monitor section
(II) intel(0): I2C bus "DVODDC_D" initialized.
Which for me looks like it only detects the VGA head and not the laptop screen. Maybe it could explain why it only works when a monitor is plugged at the back of my laptop. When it tries to detect available modes from Monitor0, it works flawlessly on my crt when plugged but it gets nothing in the other case (monitor unplugged) and bails out with a
(II) intel(0): I2C bus "DVOI2C_E" removed.
(II) intel(0): I2C bus "DVOI2C_E" initialized.
(EE) intel(0): Unable to read from DVOI2C_E Slave 236.
(II) intel(0): I2C bus "DVOI2C_E" removed.
(II) intel(0): I2C bus "DVODDC_D" removed.
(II) intel(0): Output VGA disconnected
(II) intel(0): EDID for output VGA
(II) intel(0): Output VGA disconnected
(EE) intel(0): No valid modes.
...unloading some modules...
(EE) Screen(s) found, but none have a usable configuration.
Fatal server error:
no screens found
go figure...