I don't know if dreamhost has done this recently, but I've heard many stories of people having their accounts closed due to "using storage for a purpose other than web hosting".
Yes, it is against their terms of service and I'm sure they enforce it. They now market their storage as "Unlimited + xGB" where the x GB is the amount of space you're allowed to use as personal backup space (currently 50GB).
http://www.dreamhost.com/unlimited.html