[pfg]

Why would they include something like that in VAC, instead of the Steam client? Besides, there are better ways of validating DNS sanity (just look up a known hostname controlled by Valve and compare the result to what you expect it to be). What if I just flushed my local DNS cache? Would VAC think my DNS is broken?

No, we can safely assume this is done to compare your DNS cache to a list of hosts known for their involvement in some kind cheating (be it a website distributing cheats or some kind of cheat connecting to a certain host.)

Even if Valve doesn't intend to use this for any other purpose (and I honestly don't think they are) doesn't mean that a) they won't change their mind later on and b) their infrastructure is perfect, meaning someone could steal this information.