Oded Ran, head of Consumer Marketing for Windows Phone 7, told that all Xbox games could be ported on Windows Phone 7 with just 4 lines of code. He also confirmed that Microsoft are talking with all game vendors to port their games to the new mobile platform. Games such as Doom, Duke Nukem, Sensible World of Soccer will be available at the launch of Windows Phone 7. That makes Windows Phone 7 not only a smartphone OS, but a portable Xbox, too.
Microsoft has already confirmed that developers
will be able to use the same XNA coding language to port between the
Xbox 360, Windows Phone 7 and PC titles, however it hasn't to our
knowledge confirmed that it will be so easy until now.
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Edit: Thanks to Chad Carter, who commented this news on twitter, we can say that this news is not correct. It turns out that most of the Xbox games are written in C++, but since Windows Phone 7 doesn't support native code this games must be rewrited in C#, and this porting isn't so easy. After all there are few games, that are written in C# and they can be ported to Windows Phone 7 very easy.