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Plus you have to remember that tracking services work across many sites, so they can use the correlations and timings to get a pretty good idea when a particular IP has changed users. Plus new IPs come from the same allocation group, so combined with where you visit and other attributes of your system (resolution, plug-in behaviour and so on), it's not necessarily that difficult to join the dots ... and it's not like the people using such tracking services care if they get a few wrong.
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