i once had this issue, too: turned my hd2.7
on in the morning, had a look at the display and voila - all settings are gone to factory state (brown tiles, english lanquage, all my apps were gone, too....) first time i was shocked; but then i discovered that this behaviour was the result of the non-compatibility of my sd-card! i switched my device off, pulled out the sd-card, re-inserted it directly after pulling it out and as i startet my hd2.7 again, all settings, apps and data were back!!!!! seemes to me, that any sort of cache or ram partition on the card runs into full after a few days of continouus using the device, and then the card disappears from the raid-system, looking to the user like performing a hard-reset... it's just like you would have startet your divice without the card insertet then.
for me, the solution was to buy a stated-as-compatible sandisk 8gb class 2 card - now all my wp7 problems are gone away
° htc TITAN I (vodafone) @ Dynamics v2.0
° htc HD2 (16gb) @ pdaimatejam v7.5, hw radio 2.12.50.02_2
° xda neo; xda orbit 1; htc Touch Diamond; htc trinity; Xda Touch Diamond ....