jasonr has posted over at the Steam Users' Forums details of a Steam Client Beta which is available.
This one is of particular interest because it allows you to remotely download and install games. If you leave your main PC running with Steam logged in, then you can log on from any other PC and trigger a game to be downloaded.
There are conflicting reports over on their forums that the feature is still not working, even with the Client Beta enabled. However, this is certainly going to be an ideal new feature for some, and hopefully a precursor to having similar functionality built into the Steam mobile client.