According to a news brief in Media Life Magazine today
MSN has admitted "only" about 9 million people have subscribed to MSN8 ISP service versus AOL's 35 million, despite spending about $300 million on advertising. The problem seems to be due to heavy churn from campaigns of years past, old MSN access users are dropping off the system as their heavily discounted multi-year accounts run out. MSN is planning to offer a new selection of services that don't include ISP access, you buy whatever ISP access you want and then upgrade to MSN for extra services such as IM and "enhanced email" for a orice that's under $10 a month.
What this means to broadcast emailers and list owners: Watch your @msn.com names carefully because you may see your own heavy churn matching MSNs as people drop off and start new accounts with other ISPs. It's definitely worth tracking this URL as a separate part of your list if you've got a big one.