It's certainly the largest single source of internet traffic at our house, and within that subset, streaming Dora the Explorer Season 1 is the largest single source of internet traffic coming from Netflix.
That being said, we'd use even more bandwidth with Netflix if their on demand streaming selection didn't suck for more recent releases. I don't watch a ton of older movies and those I do want to watch over and over again I own on DVD or Bluray which I would prefer to use since the quality is better and there's never any buffering problems.
iPhone + 3G + handy stand = quiet(ish) meal.