I disabled the internal screen, then I unplugged the external display…… and I got a blue screen, and I had to turn my Mac Off to get access again.
This is unfortunately a bug in MacOS X (so far seen on Snow Leopard and Lion). When the internal screen is disabled by SwitchResX, you have to turn it on manually before unplugging the external screen. MacOS X will not enable the internal screen automatically.
Worse: it will think the internal screen IS active, when it is actually not, so any tentative to force turning is on will be unsuccessful...