Valve have anounced various improvements to the Steam client, such as an HTTP download system and in-game patching.
The HTTP system will play nicer with a greater variety of firewalls “and will automatically take advantage of web-caching proxies installed at ISPs.”
The in-game patching will drive down the size of patches significantly, and will allow Steam to download a patch for a game whilst you are playing it.
Furthermore, down the line Valve says the updated system will allow it to finally implement some requested features “like download scheduling, bandwidth throttling, and prioritizing which games get downloaded first.
Thanks to PC Gamer for the info.