I know of some providers who claim to offer "unlimited diskspace", but their customers have switched to us because they had issues with slow performance, the customers didn't even need so-called "unlimited diskspace". If you truely need diskspace that is not included with realistic Web/Reseller/VPS plans, then you'd be best off getting your own Dedicated Server.
As other members have mentioned, providers claiming to offer "unlimited diskspace" usually have many other hidden restrictions that prevent you from being able to use lots of diskspace, by them restricting things like, inodes, bandwidth speeds, etc.