I randomly was looking through the internet (as you do) when I came accross this link.

http://www.realtechnews.com/posts/2088

Having sifted throught the comments and article, it worries me that in one quote someone apeared to be banned after getting a demo video (made by another player and emailed to Valve) made of him getting four headshots in a row.

Now don't get me wrong, all cheaters should go burn in hell, but there are really good players out there (as well as people who get lucky shots).

Another thing is people being banned through other means. Steam uses VAC (Valve Anti Cheat system I believe). It checks whether: a)Core game exe and dlls have been modified, and b) what programs are running as you play. From what I have seen, some programs are mistakenly labled as cheats. The steam forums and site seem most unhelpful, and also claim that VAC is 100% correct, and that if you are banned you are cheating filth. ¬¬

My advice is that when playing a game through Steam, turn off ALL un-necessary programs (such as msn, etc). Leave firewalls and virus scanners on. Never turn those off. If you do get banned, don't go to the forums, they're useless and you'll be flamed to death. Instead go to the Steam support page. HOWEVER, don't do it instantly. I have read of cases where accounts are wrongly banned, and are unbanned around a week later. Leave it a few days and see if you are unbanned.

Additionally, a VAC ban does not stop you playing these games online. You can still play on non VAC secured servers, and can still play in single-player.