normal geotiffs force you to load the complete file into memory, not very doable on mobile devices. however, you can tile them and add overviews as well, so they can be read partially. http://www.gdal.org/
has free tools for this:
gdal_translate -co tiled=yes -co compress=lzw huge.tif hugetiled.tif
gdaladdo hugetiled.tif 4 16 64
creates "hugetiled.tif" from "huge.tif" with 256x256 tiles and includes overviews at 1/4, 1/16, 1/64 size.
hugetiled.tif should work rather nicely on mobile stuff, it can be read and decoded partially just as other format map tiles. it could be a bit trickier to implement though because of
a) a shitload of different formats for projection & coordinates
b) the overviews can have arbitrary sizes, ie not a power of 2
btw, hugetiled.tif will also be A LOT faster on the PC at home when used with TTQV or GlobalMapper.