build_arch vs. configure.build_arch

[Ryan Schmidt]

What is the difference between build_arch and configure.build_arch?  
Which should we compare against in Portfiles when we want to know?  
Which should we set when we want to force the build one way or another?

Is there an easy way to say "this port doesn't support 64-bit" or do  
we have to do the "if x86_64 is requested then use i386 else if ppc64  
is requested then use ppc" dance?