@Delta6326: You don't need to put every application into the Application folder, copy it into your personal Documents folder, run it from there, might work. It installs everything into your Documents folder anyway (which is stupid), so it's likely you won't need permissions.
@Necrofire: Games that use Steam's middleware/API/whatever, are sort of ported automatically. I believe they made it originally with the plans for making it portable, so graphics calls were handled by the API, and then were sent to either DirectX or OpenGL. That explains why it works easily. There are a lot of casual games that are available for Mac, so the other available games on Steam are simply the Mac version being downloaded, rather than the PC version, both of which are on Steam servers I figure.
@NC37: Those will probably be coming in the next few weeks, there'll be new releases on Wednesdays. I assume so that they can show off new Steam features each week. I like the fact it's Portal and all, but I think multiplayer games for Steam Play would have been more effective. For example; Play a few rounds on your work Mac at lunch (when the boss isn't looking), go home, and continue on your PC. Although they might be displaying the advantage of Steam Cloud for this, so you have your tags, keyboard and mouse settings, etc all stored remotely for you.