Oho, Twitters internal IDs are moving towards 64-bit integers. So remember to update your e.g. MySQL integer columns to BIGINT.
This means that if you currently use signed 32-bit integers to track user IDs in your system, you should update your code to use 64-bit integers as soon as you can. To be safe, and to provide a date to aim for, we recommend making this change by July 1, 2013.