Looks like there's cross-platform support for it too. http://p7zip.sourceforge.net/ Linux and similar at least... Dunno about OS X so we'll have to hear back from one of those guys, but it says it supports BSD as well so it might work.
For OS X, only if you use the command line, and install Fink. Neither option is really acceptable for the majority of OS X users. I'm pretty sure that StuffIt Expander doesn't suppor it, though the pay versions might. Of course then it's the pay version and many won't do that either, at least not for this. I'm not sure how good 7zX is, but it is a GUI, and something extra to install. Most OS X users would likely use the OS X package for FS2 that already exists though so I don't know how many would care anyway.
It's something extra for Linux people to install as well so they would probably not bother just for this. If they have File Roller installed (part of the GNOME desktop) then it is supposed to support 7z so it might not be much of an issue for most Linux users. Not sure if KDE can handle it by default.
In any case (Windows, Linux, OS X) it's a new format to deal with. Some people will make a fuss, others will deal with it, and others won't even flinch.