NPD estimates XBLA/PSN downloads
Growing numbers buying digitally.
Since we're sure you can't get enough of the monthly NPD circus, you might also care to note that the market research outfit has released estimates for the number of Xbox 360 and PlayStation 3 owners regularly buying stuff on Xbox Live or PSN.
According to figures quoted by Reuters (thanks Kotaku), 18 per cent of Xbox Live Gold users regularly download from Xbox Live Arcade, while 10 per cent of PS3 users regularly buy stuff on PSN.
The article also says that 17 per cent of PC games sold in 2008 were digital downloads, thanks in large part to services like Steam. However, it doesn't say whether any of the figures above are worldwide or US-specific.