i started hitting problems with my n900 had less than 5MB in rootfs.
apps wld often crash when below that.
i stuck with that for about a few months & noticed that remaining space would always fluctuate by abt a few megs.
hope the info helps.

anyway, i've modified my n900 since then.
directories /usr/bin, /usr/share, /usr/lib, /lib, are all mounted from corresponding directories i created in emmc during /sbin/preinit.
lots more storage for me now...

kh