Check out http://www.nvnews.net/vbulletin/showthread.php?t=162171
for the 'optimus on linux - solved' thread. So far it looks like works on quite a few laptops that people have tried, including my XPS 15 L502x with a GT 540M GPU - and it should, because as far as I can see it's a generic solution that should work so long as your nvidia card is supported by the current nvidia driver.
By the way, the project was renamed to from prime-ng to bumblebee so the new link to get the install files is:
Note that on the XPS 15, the HDMI port doesn't work unless you use the nouveau driver, but you can use an external HDMI monitor if you plug it into the mini displayport via a mini-displayport-to-hdmi adapter. And I find the intel graphics to be pretty good, similar or better than my old GT 8600M card (eg scrolling in Firefox is noticeably smoother). But you need to use the 3d acceleration on the nvidia card if you want to run games in wine.