There are disputes on actual market share, but the iPod is still the market leader. 1.5 million were sold in 2003, and another half million since April of this year (when iTunes 4.5 was released). And this doesn't even include any numbers on the mini.
So, Microsoft, I would love to use WMP10. It seems really good, and to be honest I've tried iTunes a few times but was never a big fan. But if you want me to use WMP, it has to support the iPod.
I'm sure there are at least 2 million others who might agree.