If you use the Nokia Apps.xap (HTC Interop Unlock) then you can download the Nokia Maps directly from MP. When I tried updating with this file I got the error. I just got it working on my HTC HD7. The app looks like it is still in Beta. Not many options & no turn by turn.
Just noticed that Nokia Maps is for maps only. Nokia Drive is the turn by turn app. I can download both but only maps work on my Phone. When launched, Drive says Nokia Drive is not supported on your device. :-( Any help? Maybe a registry hack?