Both of the distro modes (Be or Le) are built from the same source code, but the (compiled) image needs to match the CPU architecture (i.e., big or little endian).

You can test the endianness of your system with the following "c" code:

#include <stdio.h>

int main (int argc, char *argv[]) {
   unsigned char *c;
   unsigned int num = 0;
   c = (unsigned char *)&num;
   c[0] = 0x01;
   if (num == 0x01000000)
       printf ("BIG: 0x%08x (%i)\n", num, num);
   else
       printf ("LITLE: 0x%08x (%i)\n", num, num);
   return (0);
}

more info at http://www.nslu2-linux.org/wiki/Info/EndianNess