Last time I looked there were no general DRM ebook Apps for mobile Windows; neither phone 7 nor Windows RT.
For library books try the Overdrive app
but I don't think it works for other DRM books.
There are also other specialized apps with limited use such as the Nook app.
thank you for your answers:
The Overdrive app does not work well with german books.
DL Reader only works on Windows 8 and RT, but not on windows phone 7.
Would be good for adobe to find a solution!