Thanks to Frank in the comments section for hipping us to the fact that there is now (as of April 20th) a US DVD of The Last Mistress (Une vieille maîtresse)... sort of. It's not available for sale or to rent at most outlets (including Netflix). It's a Blockbuster rental exclusive, which apparently means they have exclusive rights to it for the next three years. So don't plan on picking this up at your local mall or anything (although enterprising fans shouldn't find securing a copy too difficult, since Blockbuster sells their new release DVDs by the pound... there's already a few copies available at Amazon from marketplace sellers).
It is widescreen, anamorphic and uncut, with optional English and Spanish subtitles (and English captioning); though none of the extras from the European discs have been ported over (there's only trailers for A Girl Cut In Two, Love Songs, Inside Paris and Flight Of the Red Balloon that play before the menu). So, yeah... kinda sucks that Blockbuster has this on lock-down here in the states; but I prefer my Artificial Eye disc from the UK, anyway. At least it's finally giving the film some additional US exposure, though, as it certainly deserves to be seen.
Update 6/24/9: Despite an online rumor stating that this US disc features a longer cut of the film than previous releases, I can't find any difference between them (and I'm playing them right alongside each other on different monitors). When I heard it, I thought perhaps they'd edited the deleted scenes back into the film or something, but they have not. I've checked myself (as has Frank, again, in the comments); and the only difference seems to be the PAL speed-up. Interestingly, however, there are slight changes in the subtitles... this new version changes some ellipses to periods and periods to exclamation points. In these cases, the Artificial Eye punctuation seems to be truer to the actors' delivery.