Original Vulnerability Report: http://www.securityfocus.com/archive/1/archive/1/499042/100/0/threaded
Microsoft Security Bulletin Link: http://www.microsoft.com/technet/security/bulletin/ms09-004.mspx
The extended stored procedure, sp_replwritetovarbin, has a buffer overflow vulnerability which can be exploited to perform a remote code exploit in the context of the SQL Server service account. The stored procedure is only used for transactional replication.
There is currently a proof of concept showing that the buffer overflow is possible but there is no publicly released exploit code nor are there any active exploits in the wild. There is a workaround which involves disabling the extended stored procedure, either by denying permissions to public or by dropping the extended stored procedure from the SQL Server.