The Online Publisher's Association has a presentation that highlights some key facts to consider when talking about this topic. One such insights is that of the 18-34 year-olds who are still using a dial-up connection, the majority said that they would be upgrading to Broadband in the next six months.
Although this is quite an old study (2003) it still brings up some interesting topics, such as; how much difference does broadband (compared to dial-up) on the amount of time the user spends on the internet? And what effect does it have on the content he looks at?