The map you're looking for is here. If I remember correctly, yellow is Millenium/Broadstripe/Wave, everything else is Comcast. In shaded areas of the Comcast section, Millenium can wire individual apartment or condo buildings. For example, I live in a Comcast area of Capitol Hill but we only have Millenium in my building, most likely because they offered either the building management or the builder an incentive (e.g., wired the building for cable for free in exchange for a multiyear contract).
Every time somebody complains about their Comcast internet in Seattle, I want to smack them upside the head. Try living in an area or building that only has Millenium/Broadstrap/Wave for cable service. I dream of going back to Comcast after dealing with CenturyLink's reliable and cheap but extremely slow (usually 35Kb up, 2-3Mb down) DSL. Millenium is a non-starter with their higher prices and shitty service.