I can see private beta sites on my userpage, but not on the account tab. It seems like the correct behavior would be to hide the private beta site.
Your participation in the private beta is public knowledge: it's even advertised.
That is, all Area 51 proposal pages list the committers for a proposal (who are automatically made private beta members) and a beta site's most active users, even for private beta.
You can see the list for Movies right here:
Beyond that, the private beta's data and activity exposed to the API, and we can see everything that's going on anyway by using sites like Stack Mobile.
But I guess a more direct answer to this is, there's no point in hiding this information: if all goes well, your site, and all activity you've done so far on the site while it's in private beta, will be public anyway in a few days.