Microsoft will have quite a task ahead of them in this market, considering Apple's considerable lead and foothold. However, there are many (to say the least) Windows Mobile devices out there as well as people for whom "Windows Mobile" means business.
It will be interesting to see if and how iPhone/iPod touch app developers choose to move their products to Windows Mobile. The Windows Marketplace for mobile applications is expected to drop between September and December of this year, as are Snow Leopard and Windows 7. It's going to be an exciting autumn.