Let's try to find a good compromise regarding this. I hope the we all agree that we should not put our personal needs or preferences on the first place, it's important to improve the overall experience for most users.

First, we do not want to fail to build and install a package if you are on specific case (xcode3, xcode4, ...).

I think that most people do not have `ARCHFLAGS` env var defined and they should not even care about it. Compilers are smart enough to pick the right architecture for them (xcode3 will build ppc/i386/x64 and xcode4 i386/x64, and who known maybe next version will include armX).

As for package maintainers and they build machines, they are more likely to be already aware about what they want to distribute. For this reason I consider that they can hack the build environment to meet their distribution needs (whatever env vars they want, blend xcode4 with xcode3,...). 

Still as stated in a previous comment, we do not want publish Universal binaries that will miss PPC architecture, in order to prevent this distutils should throw an warning if ppc architecture is not supported (xcode4).

Also if the ARCHFLAGS is defined and includes PPC, the build is supposed to fail.

Some other considerations: think that "some" people may want to install packages to their OS provided Python and in this case the python that comes with 10.6 doesn't have a PPC arch.