Determine the architecture of a Mac from the command line or script?

There are many ways, but try uname -a.

arch is available in /usr/bin/arch

You can get OS version information with sw_vers

$ sw_vers
ProductName:    Mac OS X
ProductVersion: 10.6.3
BuildVersion:   10D573
$ sw_vers -productVersion
10.6.3

uname -m seems to output the same information as /bin/arch.