Monitor color space and document/working color space are completely divorced and they don't need to match. In fact they never completely do in any situation, they are two separate and different things. In a color managed display path, one is remapped / converted into the other so that the file displays correctly.
As document profile, you should always use a standard color space like sRGB, Adobe RGB or ProPhoto. The monitor profile is a different animal. It's just a description of your monitor's native color space as-is, whatever strange and unorthodox variety.
DCI P3 is a wide gamut variety with shifted primaries compared to traditional wide gamut (which is fairly similar to Adobe RGB). Why Apple chose this new cinema standard is anybody's guess, it's hard to see that it has any advantages over traditional wide gamut. But I suppose that's Apple in a nutshell...
There is nothing special you need to do about P3 as monitor space. You set everything up normally. You must, however, calibrate and profile properly, so that color managed apps like Photoshop have a valid monitor profile to work with. And as with all wide gamut displays, it only works as intended in a fully color managed environment.
Fine, thank you, then I'll work with AdobeRGB (or eciRGB as we use it in Europe). As you understood, I was confused by the presence of the DCI-P3 profile in Photoshop and thought I'd use it with retina iMac.
Photoshop just lists all profiles available on your system, and maybe OS X sets P3 up as default display profile. That will work as a generic out-of-the-box solution until the display is calibrated/profiled in the normal way.