So WB HD is up and running on Comcast in Seattle and in Portland, OR. Chicago to follow on 4/1/2004 and Philly on 6/1/2004.
I think someone mentioned above that New England (Boston) is also happening on 6/1/04.
Other Comcast systems likely to follow, depending on bandwidth considerations, (local WB stations being owned by the Tribune Company) are in:
New York (northern NJ; southwestern CT)
Also, Tribune owns Fox stations in Sacramento, Indianapolis, Hartford, CT and Harrisburg, PA, so the Comcast systems in those areas are probably ready from a contractual/legal standpoint to carry their respective local Fox digital signals (if any).