SQL Server 2005 introduces a revamped security model including users and schema separation & more granular grantable permissions. ALTER LOGIN statement in SQL Server 2005 allows you to disable the sa account and rename it. This is a good security measure to take on SQL Servers running in mixed authentication mode for example. The statements to rename and disable sa account is below:
 
ALTER LOGIN sa DISABLE;

ALTER LOGIN sa WITH NAME = [sys-admin];