That “s” is new in SQL Server 2005. It means that the SPID is a system process. For example my database is having a problem with tempdb running out of space for no reason. When I look in the log I see that SPID 118s is the offending SPID. When I watch SPID 118 using sp_who3 I see that the output looks a little strange. I see a lot of CPU and disk load and that the thread and been logged in for ever using the sa account but with no hostname, host process, or mac address listed. This is because this is the service broker process which is what runs the activated procedures. Starting in SQL Server 2005 system processes no longer need to run with SPIDs less than 50. Because of this, Microsoft has decided to make live easier when trying to identify the system processes by putting the s after the SPID number in the logs.
This same information appears to hold true for SQL Sever 2008 (so far at least).