Oh Lordy. Looks like Valve is planning to make a Mac announcement, and while it might not be quite as far-reaching as bringing the whole platform over, they might as well start with all of their blockbuster games. It's also possible that this is related to the secret message hunt going on the PC version of Portal, which itself seemed to be hinting at an announcement of Portal 2 or even the long-awaited Half-Life 3.
Whatever the announcement, it looks like Valve will have something to tell us at GDC next week. I'll be there with ears open -- if we hear that Valve is bringing some or all of their catalog to our platform, you'll be the first to know.
Update: Our game-obsessed friends at Joystiq have gotten copies of five of the images (there appear to be six total, according to the dots on the image above), and you can see them all in the gallery below. The rest of the images show the Steam platform itself, and Francis from Left 4 Dead. Awesome.
Update 2: Looks like Macworld got the last image. You hurt us, Valve. But we still love you anyway -- how can we not after seeing that "rest of us" ad parody for Steam below?