I just saw that M2TW is being distributed via Steam. So, if you want, you can download it now and play it instantly upon the coming of the 15th. Its only available in North and South America (sorry rest o' the globe). HL2 didn't take me very long to d'l, so I am very seriously considering it. The same list price applies: $49.95.
I noticed this too...then someone skillfully pointed out that the game is 11GB, thus the wait of downloading it over Steam would probably cause me to inflict bodily harm on someone.