MSN uses the domain beta.search.msn.com during Betas of new versions of the MSN Search service.
Getting referrals from beta.search.msn.com means that the person doing a search on MSN got randomly selected to use the Beta site during the Beta period. When they selected a link, the referrer to your site was MSN Search's beta URL.
Can you let us know where you got this information? I don't really agree with you as we are seeing referrals for a site that has no Overture listings for the particular keywords that are being referred from the Beta Search.