When Adsense started, I guess the hope was that a strong competitor will emerge in the next 5 years
MSN adCenter has a pretty nice program, with some elements of customization I really wish AdSense had.
The problem is, they don't have any advertisers. At least, hardly any.
Advertisers want to go where the eyeballs are. And the eyeballs are still all with Google.
And it doesn't help that Google is throwing around $100 AdWords vouchers to everyone on the planet. I've gotten probably 50 since the beginning of the year (mostly for me to give to clients) I don't see Microsoft trying to entice advertisers this way (yet)