Looks like a P22 handwriting font. Searching now....

They may use P22's Dearest Script on their website, but I believe the logo is custom (no hits on any of the databases):
http://dporthaultparis.com/about/
http://www.myfonts.com/fonts/ihof/p22-dearest/

It's not custom...I've used it before

Ben, luxury brands rarely use an off-the-shelf typeface—it makes it too easy for counterfeiters to knock off their products.

Someone else may recognize it, but I'm sticking with my original assessment until proven otherwise.

I found lots of very similar typefaces, but no perfect matches—even searching on individual letters on three separate type databases.

Hiatus by Stephen Rapp comes close, and showed up in search results of several letters:
http://www.myfonts.com/fonts/stephen-rapp/hiatus/

I spent a little time researching this via trademark registration. Here is the earliest version of the logo I could find—it was in use from 1948 to 1996:

Here's the mark from its 1984 Canada trademark registration:

Here's how the mark looked on the company's website in 2003 (via archive.org):

And again in 2004:

The company was sold in 2005, and I could get none of the archive.org links to work for 2005, 2006, or 2007. February 2008 is the first instance I could find of the logo in it's current form:

Based on this progression, I think it's pretty clear that the logo has been redrawn and refined from earlier versions (the most noticeable difference over the last decade being the removal of the horizontal foot on the P and the redrawing of the crest). I suspect the original was based on Daniel Porthault's own signature (or one drawn by his wife Madelaine, who was the creative force behind the company).

These have the overall form, if not the nuance, of the Porthault logo, and may share a similar source—French cursive or roundhand writing:

http://www.myfonts.com/fonts/archivetype/archive-french-script/

http://www.myfonts.com/fonts/archivetype/archive-roundface-script/