FBReader will today read DRM-free MOBI. It has several issues with MOBI though, for example: a) no cover image, b) much of the layout is ignored, c) high compression is not supported. I think FBReader's MOBI is "good enough", but not everyone agrees. It could certainly be improved, and it would not take much effort to do so.
The other approach is off-line conversion to ePub using Calibre, once again only for DRM-free MOBI. This has fewer limitations than FBReader's MOBI, although it means you will end up using Adobe's ePub viewer which has limitations of its own. An advantage of pre-processing though Calibre is that you can customize the layout to some extent.
is an implementation of Calire's ePub and PDF to MOBI conversion capability on the Kindle 2. Which demonstrates that is should be possible to add Calibre's MOBI to ePub conversion directly on the DR1000S.