While I'll concede that it's still in Beta, there is already a virus targeting Windows Vista's command shell.
Written in July, the viruses take advantage of a new command shell, code-named Monad, that's included in the Windows Vista beta code.
The viruses were published last month in a virus-writing tutorial written for an underground hacker group calling itself the Ready Ranger Liberation Front, and take advantage of security vulnerabilities in the new command shell.
However, it appears that the shell may not even be included in an actual release of the OS. See report here.