I recently got a Dell L502x laptop. In order to connect it to an external VGA monitor I am using the mini-Displayport (a la Apple).
The problem is that Linux (Ubuntu Natty) does not always figure out the right display modes for this particular monitor. Unfortunately, cvt nor gtf generate the correct modelines and I am stuck with a lower resolution.
The trick was to dual-boot Windows :S and use PowerStrip to extract the proper modelines, as explained here http://www.x.org/wiki/FAQVideoModes
A bunch of very useful information can also be found here: https://wiki.ubuntu.com/X/Config/Resolution