Well, not officially called Nook for Windows Mobile...
For those with Windows Mobile phones and want a NOOK app.
This is the app that came with my phone (HTC HD2 Windows Mobile 6.5). Don't know if it'll work for other Windows Mobile phones. Give it a shot.
images.barnesandnoble.com/PResources/download/WinMo/BNeReader_1.1.0.22.CAB
The app is awesome! Great feature set.
Note: I'm referring to Windows Mobile not Windows Phone.
Edit: Here's an old XDA Developers thread for the B&N eReader app (Windows Mobile):
http://forum.xda-developers.com/show...=650889&page=8
The last post is from August. Could be helpful if you need to troubleshoot and need help.