You are right, it seems that about 80% of internet users had broadband in 2008. However, about 40% of people in the US were still not users of the internet and 10% were still on dialup. I don't know if the average speed means much in terms of addressable market for streaming (if 1% of the people in the US have gigabit connections, the average is 10mb even if 99% of people are on dialup), but I couldn't find any statistics on the median internet speed.
