Twitter has issued its first transparency report, covering governmental requests for user account data for the period January 1, 2012 – June 30, 2012.
Out of the 849 requests it received during this period (a number significantly lower than what I had imagined), 679 were from the U.S. for user account information on 948 users. Of those 679 requests, 75% resulted in Twitter providing some or all of the requested data.
See the report, and kudos to Twitter for disclosing these numbers.