It appears that the first Windows Phone 7 ROM may have leaked onto the Internet. The 100MB file apparently comes from an HTC device called the HTC Mondrian.
The leak is very recent and precious little information has yet been revealed about the HTC Mondrian. The ROM shows the following:
NBH_WM7_AKU_6176_WWE_for_HTC_MONDRIAN
Model: HTC Mondrian (Cingular USA)
AKU: WindowsMobile_7_AKU_6176
Language: 0409_WWE
Hopefully we will learn a lot more in the next few days as people begin to dig into this ROM.