Using external monitor when lid is closed

Hi
I'm using a 16:10 Dell Monitor at work.
When connecting the 16:9 XPS you get a reduplication of the Unity pannel or gnome background on the bottom of the screen (the 16:10 factor)
you can't adjust the same resulotion so you need to work with the lid closed.
This works great but if you reboot the screen becomes blank and you need to open the lid to get the display working and then close it again. This behaivour is not common to other Dell laptops I worked with running windows So I guess this is not the best behaviur posibble.

Regards
Roi Illouz