(Pardon me while I throw up.)
(OK, I feel better now.)
I have never understood why some people insist that cookies need to be a measurement of web site user populations. They were not created for that purpose, they were never meant to be used for that purpose, and any attempt to use them for that purpose is likely to fail, while exhausting resources that can be more productively used (like fixing bugs).
There are already many types of techniques that can be used to statistically estimate a user population.