Originally posted here by SirDice
Correct.

Seems to depend on the OS and the version of nmap.

RTFM? Or read the tutorials right here on AO

But... If a client has a firewall running you probably won't detect anything.. The firewall will block any and all incoming connections.. So this probably will not help you..
Are the tutorials you speak of on the main site or on the forum (via searching)? Also, while I recognize that most firewalls won't respond to most stealth scans, what about firewalking techniques (using non-RFC compliant packets)? Are those still popular?