One thing the comments here haven't addressed is how Twitter's behaviour in this case compares with how such things have traditionally been dealt with on the Internet. I think there's a reasonable expectation that big Web companies act in a way compatible with the rest of the Net even if they are for-profit.
Now obviously girl-geeks is not a personal nickname, nor apparently was it previously the name of any sort of registered company or other entity, more of an ad-hoc sort of thing. Given that someone later formed a company with this name and trademarked it, should they get the rights to that Twitter name? Would they be able to take girl-geeks.com off a previous owner who was using it in good faith?
My suspicion is that they wouldn't, and Twitter's behaviour is bad netiquette.
Consider the Nissan Computer Corporation website: http://www.nissan.com/
Or read about when a company called GAIL tried to take gail.com off a woman named Gail: http://www.wipo.int/amc/en/domains/decisions/html/2006/d2006...
Now obviously girl-geeks is not a personal nickname, nor apparently was it previously the name of any sort of registered company or other entity, more of an ad-hoc sort of thing. Given that someone later formed a company with this name and trademarked it, should they get the rights to that Twitter name? Would they be able to take girl-geeks.com off a previous owner who was using it in good faith?
My suspicion is that they wouldn't, and Twitter's behaviour is bad netiquette.