The word according to pocket now

is that XDA Developers have already dumped Windows Phone 7 build 7.0.0.6077.

The article says that this is the starting point for all work that goes towards porting to mobile devices and towards making custom ROMs, although they foresee a long road ahead.

Windows Phone 7 build 7.0.0.6077 comes via the Windows Phone 7

Emulator for PC with the ROM image build dated the 2nd of March 2010.

The reason being is the lack of an actual ROM for an actual mobile handset may well be close to impossible; however it can give precious info about the Windows Phone 7 Series platform and how it is built and working.