Interesting interview here which, among other things, suggests that Half-Life 2 will use aggresive anti-cheating techniques.

You're not talking about multiplayer modes as such, but can you tell us something of your plans for anti-cheat measures in Half-Life 2 online?

Lombardi: Steam.

So how will that work?

Lombardi: You're constantly connected, and we can constantly touch your bits. Steam allows us to do an arbitrary number of updates per day/per hour/per minute, so if your code base is constantly changing then it's hard to write a hack for that code base.

So essentially, anyone playing Half-Life 2 online will have to do so through Steam?

Lombardi: Right now, when you go online, you make a couple of handshakes to make sure you're an authorised player who's actually paid for something, whether it be Half-Life or Counter-Strike or any one of our games. And then you're being pointed at a server list, etc