I think if you're playing with people using a mac it should have a little apple symbol by their name - that way you can single them out to fully abuse them

Good idea, although most of them will refuse to leave the spawn room until they get the right hat anyway

One thing I wonder - is that I know that the source engine is pretty awesome and all that (and come a long way since Half Life 2 - but isn't it getting a little dated now?
Well a lot of the 'new engine' talk that always flies around is marketing bs anyway. I'd imagine the biggest issue with an osx conversion was supporting OpenGL rendering (assuming they've had to go that way - I'm not a Mac guy). I always get the impression that each Valve game forks the engine and has their own version, rather than their being a proper 'shared' engine base ; for example, L4D1/L4D2/TF2 all seemed to have had different levels of multithreading support. If that's the case it's a fair bit of work, but far less than rewriting the engine from scratch (this hardly ever happens, I don't care what these companies say)

As for stuff like DX10/etc/etc support, you can be sure it's there internally already for HL3, but there's no real advantage in enabling it. You'd just get more support hassles for fairly minimal gain in sales.
Edit:This might be my geekiest post ever. So so sorry.