In my opinion, pairing the Windows Phone with the Zune Marketplace/Zune Pass and making some sort of Zune application will add sales to both divisions. Right now, I have a Zune and WinMo6.5 phone. I can't put ANY of my Zune Pass music on the phone, which is strange b/c they are both Microsoft products. I think Apple's marketshare can at least partially be attributed to how well all the parts play together. Further, the Zune UI is great and the Windows Media Player...is...uh....not so much great.
Please, please, please make a Zune application that supports Zune Pass on the Windows Phone. Did I say please? OK, don't wanna beg here, but...
AT&T Tilt