Yocto 1.2 warns the user if they are building on, for example Ubuntu Precise 12.04. The sanity check that warns users should be updated now that 12.04 is out. The same should apply to Fedora and OpenSUSE if they release something before 1.2.1 comes out.
Fixed in poky denzil as of: http://git.yoctoproject.org/cgit/cgit.cgi/poky/commit/?h=denzil&id=119e1b7dc97a82ff0791d1b4ba519361ff46cbd1