Running Debian Sarge on an HP pavilion a302x.
I can turn off the fan by doing:
modprobe fan; echo 3 > /proc/acpi/fan/FAN1/state
but then doing:
echo 0 > /proc/acpi/fan/FAN1/state
does not turn it back on; it has to be rebooted to get the fan to start again.

I changed these lines in drivers/acpi/power.c:

-       if (resource->state != ACPI_POWER_RESOURCE_STATE_OFF)
-               return_VALUE(-ENOEXEC);
+       if (resource->state != ACPI_POWER_RESOURCE_STATE_OFF) {
+               ACPI_DEBUG_PRINT((ACPI_DB_WARN,
+                       "Device [%s] says it's still on\n", resource->name));
+               resource->state = ACPI_POWER_RESOURCE_STATE_OFF;
+       }

and now it works. This indicates that the hardware is lazy and doesn't bother
updating the state to indicate that the fan is now off. Linux therefore doesn't
turn the fan back on because it looks like it's on already.

A possible solution would be always to carry out actions even if they appear
redundant, perhaps printing a warning message.