You can upload and share privately on SlideShare. You need a SlideShare PRO account to do this (from 13th Jan, 2011).
When you upload privately, you can keep it the content completely private so that it is only visible to you when you are logged into the site.
With private uploads, you get six options for sharing with others:
Schedule Private Presentation to go Public
To know how each of these options work, browse through the above presentation.
If you have a basic SlideShare (free) account, you cannot make private uploads from 13th Jan, 2011. You will need to upgrade to a SlideShare PRO account.
If you have a free account and you have uploaded privately previously, they will continue to remain private without any disruption (if you change from private to public, you will not be able to change this back to private though).