There is no sure fire way of detecting and tracking a unique visitor, that's just the nature of how things on the web are done. I think we're all in agreement with that. However one particular issue that I could see cropping up here is something like when a certain big ISP who shall remain nameless rotates their IPs sometimes on a page-by-page basis. Obviously an extreme event but in that instance, the hash would be different and the visitor would be deemed "unique". Then again, that example shows quite clearly why you shouldn't rely on an IP. :)