Source: Steam Bans
Please remember this is the opinion of someone else...

For the past 6 months or so, people have spoken about the Valve Anti-Cheat version 2 with great expectation and excitement. Lately its been announced that its going to be very effective, but the truth is different. It needs more work!

Valve takes many precautions in their anti-cheat department, which leaves alot of people with nothing else then trying to figure it out themselves. They are notoriously silent about their system, and the argument that it would only give cheaters indications that will help them crack it could be valid, but its not. Cheat-developers almost knows more about the VAC system then Valve themselves, and keeping information will leave to nothing but faint rumours and peoples wild imagination. This means that steambans.com hasn’t been able to retrieve factual information on the system, but we have done some research.

First, we established that changes came in the system along side the announcement that VAC2 beta will arrive soon. There was changes in server modules that suggested a rather significant change in parts that haven’t been changed in a long while (VAC - core hasn’t been changed since steam platform was initiated). Secondly reports that on selected servers, mainly CS:S servers people were forced to download additional information before connecting. Additional downloads that they were not required to do elsewhere on source servers, and downloads that most definatly wasnt due to custom content.
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