Updated at the bottom.
This very hostname reports important information that has been overlooked. First, the "wc" in wc09.mtnk abbreviates web-cache; this IP is web-cache no. 9 of a bank of caching proxy servers in Newkirk Road, Thorn Hill: mtnk presumably abbreviates Metro Newkirk. (Rogers has an office there.) Web-caching proxies are used by Internet companies such as Rogers to save bandwidth -- copies of web-pages are stored on Rogers' proxy, and only if a page is not stored there does traffic get forwarded to a website; when the traffic gets there, however, the IP that is logged is that of the proxy, not of the original requester.
126.96.36.199 is one of 42 such proxies in the Rogers stable, and they were (are?) arranged in three banks of fourteen at Wolfedale in Mississauga (abbreviated to wlfdle), York Mills in Toronto (ym), and Newkirk in Richmond Hill (mtnk) (see here). Rogers routed all its traffic though these servers -- or at least the traffic of all users who used the proxying function. (An old Rogers FAQ, quoted here, encourages its use, but also gives instructions on how to turn it off if it isn't helping.)
That 188.8.131.52 was a web-caching proxy explains a lot. It explains, for example, why it is so easy to find the IP being used in these months: dozens of different users of this IP can be identified for every month, including September, October, and November 2003, when the Cools post was made. For your own curiosity, take your own IP (it is listed in the widget to the right) and google it. Do you see any traces of your own surfing? I can't find any of mine. Why are hundreds of instances of 184.108.40.206 so easy to find? Because as a proxy it was through-putting hundreds or thousands or tens of thousands of times more traffic than an IP assigned to an individual subscriber.
This also explains why we find such a wide variety of individuals using this IP in the months that the Cools post was made. Some of them could read Vietnamese (here and here), Tamil (here), Korean (here), and Danish (here). And they seem to come from a wide variety of locations in Ontario: London (here and here), Waterloo (here), Aurora (here), and (as we know from the present controversy) Ottawa. Again, this is because it's a proxy -- and these are all areas served by Rogers and so any traffic from them might be routed through one of Roger's proxies.
That 220.127.116.11 is a proxy is clear, as is the fact that there seem to be only 42 such proxies to serve all of Rogers' customers (they're listed here).
Although the proxies can change, they do not seem assigned randomly, either. The IPs associated with the prolific editor of numerous Thomas the Tank Engine articles in wikipedia (here, here, and here) seem to suggest that a Rogers subscriber had a "home" proxy; that he might be shifted by Rogers to another proxy for load-balancing, only soon to return to his proxy "home".
So, how many potential Rogers customers might have made that racist Cools post? Probably all of them. As we have seen (here), the proxies are not geographically limited; each seems to be able to serve all areas of the province.
Now, according to this, Rogers had 800,000 internet subscribers in March 2004. The same link states that 90% of cable subscribers are in Ontario, which implies a pool of about 700,000.
The Cools poster could be almost any one of them.
Update. The best previous attempt to explain this matter from a technical perspective was that of Lance at Catprint in the Mash (his work was copied, pasted, and embraced at FreeDominion). He has now withdrawn his explanation in favour of mine here, and in the comments to this post.
Update 2. This post establishes that 18.104.22.168 was a widely used proxy, thereby removing the circumstantial case against Warman. For a discussion of evidence that shows that it is someone else, see Why Warman is probably innocent and Why Warman is probably innocent, part 2: Rogers Hi-Speed Internet.