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I looked at the code and the problem is here:

        def _execute_child(self, args, executable, preexec_fn, close_fds,
                           cwd, env, universal_newlines,
                           startupinfo, creationflags, shell,
                           p2cread, p2cwrite,
                           c2pread, c2pwrite,
                           errread, errwrite):
            """Execute program (POSIX version)"""

            if isinstance(args, types.StringTypes):
                args = [args]

            if shell:
                args = ["/bin/sh", "-c"] + args

            if executable == None:
                executable = args[0]

You can se that if shell is true (as it must be if one specifies args as a string) 
and executable is supplied, one ends p with a mess, i.e. the executable ends 
up trying to execute  ["/bin/sh", "-c", args]

Fortunately cwd offers a hackaround. Still, I hope this can be either fixed or 
else can produce a warning.

While on that subject, should specifying args as a string without specifying 
shell=True should produce a warning?

Thanks for subprocess. It's a wonderful module. I am really looking forward 
to using it (once a few kinks get worked out).