I'm running natty on a 13" macbook pro, and when I unplug the AC power sometimes, the monitor turns off. Sometimes I can unplug the AC adapter and nothing happens, but as soon as it starts happening it happens every time I unplug. When I plug the adapter back in, the monitor comes back.
Pommed is misconfigured by default to set brightness to 6 (out of 44000) on battery for nvidia. The pommed config file is /etc/pommed.conf, and the section is sysfs_backlight/on_batt. Setting this to a more reasonable, value such as 22000 fixes the issue. You can mess around with values to set it to by running
Note that's kind of pseudo shell code, the value for driver will probably be the only subdir or backlight, so just hit tab in the shell to autocomplete it, and the value to echo can be whatever you want so long as it's between 0 and max_brightness. Setting to 0 will turn off your backlight so I can't really recommend you do that.