We know for a fact that Microsoft was using the Toshiba TG-01 aka Tsunagi as a developer device for Windows Phone 7. Heck, we even have one of the rare birds in possession (see above). The device was ahead of its time when released over two years ago, sporting an ultra thin body, large LCD screen and a 1GHz CPU. But fast forward to 2011 and it's quite the clunker.
Now over at XDA, developer Nokser has been successful in porting WP7 to that device for those who only have Windows Mobile 6.5 on board. We say successful, but it is full of caveats and lots of early bugs, so this is more a proof-of-concept than a ready-to-install for the masses release. Still, it's a big step in the right direction, even if WP7 is pretty awful on it due to the Tsunagi's resistive touch screen.
Some drawbacks, should all two of you be interested are: no camera, potentially very low memory, poor reception, inability to zoom in on maps and photos, etc. These issues are expected to be fixed, so we'll see where all of this goes soon.
Source: XDA (thread); Thanks, Ross "H3ALY" H, for the tip!