There is a decent chapter on techniques used by those intrested in gathering information about your systems at It's from a New Riders book called NEtwork Intrusion Detection: An Analyst's Handbook by Stephen Northcutt. He is a SANS speaker and author, and team leader for the Dept of Defense Shadow Intrusion and Detection team. Sound very stealth, but I have no idea wht that really means. It's a decent chapter and leads me to think the book would be worth the read.