When you create a dashboard you have three options for permissions. The permissions are connected to Manage Users and the type of users that have been set.
Shared with everyone
Shared with admins (viewable by admins & account owner)
Only visible to you (only you and the account owner)
If you'd like to edit permissions for dashboards go to Settings > Manage Dashboards.
Manage Dashboards also allows you to:
Rename your dashboards.
Delete a dashboard.
Copy a link to share your dashboard with others outside your organization.