The latest advertising figures for the online TV industry have been released by media agency Starcom.
Their estimate (from their media buying activities) for the internet TV market for 2007 was around $1.3bn, with $120m, or almost 10% of spend, going to the four top US TV networks.
Without doubt, internet TV is appearing on more and more media buying schedules, but the slow takeup of online advertising is being replicated - it took 5 years for internet ad sales to become a serious revenue generator and it still accounts for a disproportionately small amount of the overall media mix.
The traditional TV market in the US is considered to be worth around $75bn, so around 1% of it is now being spent online. I don't have the figures, but I bet the proportion of TV being viewed online is far higher than this and rapidly growing.