Silverlight4 provides an option to run at elevated trust http://www.silverlightshow.net/items/Silverlight-4-elevated-permissions.aspx
Does anyone know if it also provides certain degree of elevation on MAC OS X?
Yes, it does provide a degree of elevation. For instance, you should be able to access the user's files without using the OpenFileDialog. You will only be allowed to access the current user's documents though; you won't have full access to the hard drive (this is the same on Windows).
Obviously COM is a Windows-only feature. Hopefully a future version of Silverlight will provide a similar construct on the MAC.
Here's a list of known differences of Silverlight on the MAC.
http://justinangel.net/#BlogPost=SilverlightIsnTFullyCrossPlatform