For Sql Server DBAs, sp_who is an indispensable diagnostic tool for researching, monitoring, and troubleshooting. For example, you can use it to iterate through all sessions to a particular database, logins used, and host name where a session is initiated from, etc..
sp_who is modified slightly in Sql Server 2005. It now returns one additional column called request_id. I found this out because I have a kill session script that uses sp_who. The script builds a temp table that stores sid that connects to a database, then iterate through the table to kill all the sessions.
I actually don’t know what this field is for. But if you have scripts like mine, you will need to modify them a bit to accommodate this new column.