Well, I dug into autoconfig docs and figured out configure options to set architectures and SDK for OSX.
--with-macosx-archs="i386 ppc x86_64 ppc64"
just include a list of architectures you want (above is quad-arch).
It also checks GUI toolkits (tcltk, wxpython) for available architectures, and substitutes them in makefiles as needed, so you can build a 64bit arch for GRASS, when you only have a 32bit wxpython (just make sure to specify both 32 and 64bit archs in configure, so 32bit GUI stuff can link to GRASS libraries).
--with-macosx-sdk=/Developer/SDKs/MacOSX10.4u.sdk
Necessary when compiling on a PPC Mac with OSX 10.4, or when compiling on a later system for an earlier system (like I do for by binary distributions).
No more need to add arch flags and isysroot to CFLAGS & friends 
Try it out. I tested on Snow Leopard (Xcode 3.2). I don't think I broke configure for other platforms.
-----
William Kyngesburye <kyngchaos*at*kyngchaos*dot*com>
http://www.kyngchaos.com/
"History is an illusion caused by the passage of time, and time is an illusion caused by the passage of history."
- Hitchhiker's Guide to the Galaxy