I just switched from a 20" Acer to a 21.5" Lenovo L215. The old Acer's default resolution was 1680x1050. The L215's is 1920x1080. However, whenever I turn on Ubuntu, it defaults to 1680x1050. (my graphics card is a Nvidia 9800GT with proprietary drivers) I even used Nvidia's setting tool; it would set it perfectly (even writing a xorg.conf file), but once I rebooted, back to 1680x1050.
Here's my xorg.conf: http://pastebin.com/f74a3dd8a