They seem to think so in Boston. The Suffolk County District Attorney has subpoenaed Twitter for "All available subscriber information, for the account or accounts associated with the following information, including IP address logs for account creation and for the period December 8, 2011 to December 13, 2011.” The "information" includes two hashtags, #BostonPD and #d0xcak3.
Technically they are not serving the hastag, but what does this mean? They want the IP address of everyone who posted a tweet with those hashtags? I get it, they are investigating threats to the police, that's fine, but seeking the IP address of everyone who used those hashtags during a 6 day period?