On SQL2005 you can get this from [font="Courier New"]select * from sys.dm_exec_connections[/font] in the client_net_address column assuming that their net_transport is TCP.

On Sql2000 it is a challenge. It appears that for most remote connections, the first connection/session from that client will have the MAC address coded in the [font="Courier New"]net_address[/font] column of [font="Courier New"]sysprocesses[/font]. Than you could use [font="Courier New"]xp_CmdShell 'ARP -a'[/font] if your SQL server is also a domain controller to list the current cache of MAC to IP mappings. If it is not a domain controller, well ... we are already beyond the limits of what I know. Someone else will have to take it from here.