Just to save anyone else the hassle of looking for this, here is the response from VMWare:
> Unfortunately this feature is not functional and is unsupported at
> the present time. The option appears greyed out because it is not
> possible to enable it right now.
>
> I accept that it would be better if the option did not appear at all,
> as it is misleading in that it suggests that there is a way to enable
> it.
>
> I believe this will be resolved in the next version of the product,
> (VI 4.0) which will be released next year.