The mp-fremantle-xxx is a sort of virtual package to get all dependencies for a proper maemo installatiion.
The xxx seems to depend on your location (us, uk, other europe...)

When I played with the overclocking kernels I got into the same problems and could not upgrade to PR1.2. So in the end I just had to reflash the whole box.

By installing the generic one you may just have 'converted' your n900 to an other 'region'. Don't know if this is harmfull.

you might learn a bit more from looking in the files in /var/lib/apt/lists