The other thing is, you can ping an IP and find out if someones PC is turned on (which has problems as already mentioned). This is asking to find out when people are at the PC, but not online or shown as offline in messenger. The reason the option to be shown as offline is their is so that people can go about their business without being interrupted. This would also give away if someone had blocked you. You could always plant a virus that reports back to you
(BTW, I'm KIDDING... I did that once and it got found sending me data so I was in a
bit of trouble
)
PS - That was before the
MAJOR jump in the number of firewall users
/ 
