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Although Twitter didn't specify a date, a spokesperson said they are planning to release usernames that are either inactive or have been deleted by their former "owners." Right now, policy says that no updates for six months qualifies an account as inactive -- but thus far those idle usernames have stayed dormant and unavailable.
Soon though, the sludge pile will be "recycled" and released back to the public. This could result in a virtual stampede as individuals clamor for some of the more coveted Twitter "real estate" now locked up in the dead pool, among them some short and catchy usernames that were claimed early and later abandoned.