That's a really good idea! You could also put (with server side scripting) the users' IP, machine name, user name.. all kinda good stuff.
What would also be great is to setup a little SMTP service to accept incomming mail (not do anything with it). You'ld need a service though to monitor the 'drop' folder of the mail service for messages addressed to your specially created addresses and notify you of when something has been address to one.
You'ld know who it was, when they snaked the address... but, the spam could be sent out days after the mail address was collected...