[ruby-core:26161] RbConfig::...[prefix] not sensible on OS X 10.5?

Today, I found this points to a nonsensical location.

    require 'rbconfig'
    ruby = File.join(
        RbConfig::CONFIG['prefix'],
        RbConfig::CONFIG['ruby_install_name'] + RbConfig::CONFIG['EXEEXT'] )
    system "#{ruby} -e 1"

prefix is /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr
but should be /usr/bin. The interpreter needs a way to find itself
without trusting $PATH because there are multiple interpreters on the
same machine.

Or... is it sensible to stash the original C argv[0] somewhere? Right
now we seem to ignore it.

Josh