I would like my application to make use of the StartMenu jumplist and I would like to know whether the items are somehow visible to other PC users as well?
The jumplist information is stored in the users profile. No encryption takes place, but the normal file level access rules apply. Other users on the systems can't access this information (unless they are an administrator)
Of course nothing is stopping someone from taking out the drive and reading its contents (or booting from a LiveUSB) - but for normal scenarios you can treat it as private.