I have the XPS L502x myself, with the Nvidia GT 540M. =)
Installed BT5 KDE 32 with dualbooting Win 7.

Having great problems trying to install Nvidia drivers. Startx wont start, getting "No screens found, No devices detected and ddxSigGiveUp"..
Deleting the xorg.conf lets me start though, but it aint using the nvidia drivers.

Have you gotten this to work?

Cheers. =)