SEO functioanlity on a multisite

Hi

We have a multisite setup, however at present are only running 1 domain / site. We now want to create a new site and obviously easiest way will be to make one as a sub-domain of our existing site - eg. http://newsite.mydomain.com

As the proposed new site is going to be totally unrelated to the main domain (just a private corporate site with shareholder info, for which we do not want any SEO), our question is whether this sub-domain will still get connected / ranked by search engines etc as being related to our main domain, which we do not want.

If so, we can obviously create / host a stand-alone site elsewhere for this purpose, but just wanted to run it by you first and hear your advice.

Thanks,
Paul