[drummer32]

I think they already do. You begin the download and after it has downloaded some portion of the game you can launch it and the stream the rest.

[albertoleal]

I didn't know about this. Which games?

[fletchowns]

Blizzard games also do this, you usually only have to download about 25% to start playing

[KJasper]

Not Steam, but Ubisoft did this for instance with the latest Far Cry and Assassins Creed.