I own a main site that has "unlimited domains" on the hosting plan. I take advantage of this, and have around 20 or so domains hosted on the one account. All these domains can be accessed through http://subdomain.domain.com as well as by itself as www.subdomain.com. The hosting is surfspeedy if that matters... So... will all these addon-domains have to go through the sandbox? Do subdomains generally go through the sandbox or are they treated as part of the original site somehow?
No. All of your domains will be treated individually. I have the exact same type of plan thru Blue Voda.