My installation of wordpress mu uses https. Problem is the latest version of slideshare inserts a dashboard widget which has hardcoded http links. Enter mixed content security warnings...
The same is true on the slideshare plugin settings page, which has a link to http://cdn.yoast.com/theme/yoast-32x32.png
Could you please use http/https img src according to the $_SERVER['HTTPS'] setting. Even better would be to make the dashboard widget optional :)