Microsoft is rumored to come out with their own Smartphone in early 2009. According to a story on The Inquirer Microsoft is working with NVIDIA to build a smartphone based on the Tegra platform.
The new Microsoft Phone would be unveiled at the 3GSM in February.
The biggest arguments against a Microsoft phone are Microsoft's partners that make Windows Phones. They would not be happy if MS is going around selling their own Windows phone just because Apple does it.