Valve has release a new Steam Beta client that supports a remote download management function. Sweet!
The new feature basically lets you trigger a download on your PC Steam account using the internet browser on your phone via the Steam website, which will help you spend even more money on spontaneous online purchases. Sigh...
To download the Steam Beta client, go to File -> Settings. On the Account tab press the 'Change...' button to open the Beta Participation dialog. Then Select 'Steam Beta Update', and allow Steam to restart itself.
Other new features in the beta include "reduced excessive system memory usage while downloading updates in some cases", and "added debug code to diagnose download issues".