Valve has a strong message for Source Engine mod makers: stick to the official SDKs and don't use leaked or stolen code. Something that should have already been understood as not okay.
Steamworks' documentation has been updated recently with some clarifications around what is and isn't allowed when distributing Source Engine mods based on Valve's games. There's a few notable changes and additions that are probably of interest to anybody making Source mods.
Firstly, Valve expects most mods to be non-commercial creations. Essentially, they are mostly non-revenue generating for the developers. Though soliciting donations is okay, no content should be locked behind a donation or payment. It's also mentioned that in very rare circumstances will Valve allow mods to be sold on or outside of Steam and that it should not be the expectation for a project.
Perhaps the most important update is the very clear instruction for mod makers to avoid the use of leaked or stolen code. I think this should have already been obvious to most creators but now it is official. No matter one's opinion on open versus closed source, one should respect somebody's chosen license. The same goes in the opposite direction when using GPL code.
In rare cases Valve has given source code to mod teams wanting to do things not possible in the official SDKs. But they also mention that this is very much the exception and not the rule, due to risk of leaks.
On a slightly cheerier note, if a mod does eventually become a commercial product, the licensing fee for the Havok physics engine is no longer required. This is due to an agreement with Microsoft who have decided to waive the licensing fee. In the past a $25,000 license was required before putting the game/mod up for sale.