I think the Scribd link Amanda posted covers this issue pretty well. Magic Hat has no problem with West Sixth using the "6 with star" logo as long as they consistently use the words "West Sixth Brewing" as well.
There are really two issues here, copyright, and trademark infringement. The standard for copyright infringement is "substantial similarity," while the standard for trademark infringement is "likelihood of consumer confusion." Consumer confusion increases when the two companies are in the same industry, selling the same goods.
True, these are subjective issues, and who knows how the court will rule, but do you really want to spend a fortune arguing these issues in court? The logo isn't even that good. . .
Here's a quote from Judge Learned Hand “no plagiarist can excuse the wrong by showing how much of his work he did not pirate.”