YEEEEEEES, I got it.

Just forgot to add ext2 modules to the kernel:
/ # insmod /mnt/initfs/lib/modules/current/ext2.ko

Now it works fine:
/ # mount /media/mmc2

As can be shown in these results:
/ # cat /etc/mtab
...
/dev/mmcblk0p1 /media/mmc1 vfat utf8,uid=29999,shortname=mixed 0 0
/dev/mmcblk0p2 /media/mmc2 ext2 errors=remount-ro 0 0

/ # df -h
Filesystem Size Used Available Use% Mounted_on
...
/dev/mmcblk0p1 468.5M 64.0M 404.5M 14% /media/mmc1
/dev/mmcblk0p2 495.1M 13.0k 469.5M 0% /media/mmc2

This is GREAT. But now comes the big question:
How can I use this ext2 partition to extend my rootfs. That is, I want to install apps in my MMC, not in my 770 internal memory. I think it can be done by linking files and directories to the 770. But could somebody explain me how?

BTW, I've also read something about "unions"... What's the best option?

Salut.
Sebas.