Alecsandru, search is your friend:

http://wiki.maemo.org/Changing_the_eMMC_filesystem
http://wiki.maemo.org/Repartitioning_the_flash
http://talk.maemo.org/showthread.php?t=73478
http://talk.maemo.org/showthread.php?t=82175

(True, I searched for "home ext3" but a small update to the instructions is an easy exercise for the reader. BTW the last one also deals with the camera application which apparently some people had problems with.)

FWIW, I've fiddled with ext4 myself and it worked pretty well but the next BackupMenu restore reformatted opt back to ext3. I did not feel like fiddling with BackupMenu so I left it at that. ext4 might be faster but at a price: it achieves speed mostly by lazy writes which is not always safe if you unexpectedly reboot.