Latest research on file-sharing is gonna add fuel to the fire - a PhD student at Chicago finds file-sharing negatively impacts music buying by ~ 10%. Very interesting stuff - his method is to control for all sorts of variables, and regress net access with respect to time against music buying. He finds those with broadband connections using P2P are 30% less likely to buy music.
Of course, these character are generally all found in the demographic that thinks major label music sucks...so you may take this with a grain of salt or two.