Since after a few experiments i had to poweroff my n900 pulling out the battery, i have discovered that camera-ui could not save anymore the photos in the sdcard returning the message:
"Memory full. Unable to save contents."

Digging aroung i have found what is going wrong with:
root@N900# gconftool-2 -a /system/osso/af/mmc
internal-mmc-corrupted = false
mmc-corrupted = true

While the sd card is working fine because i manually checked the fs just after the crash. (and the N900 has been already rebooted a few times)
Now i have no idea about when that flag should come back to false...
But in the meanwhile, if someone hits the same problem, the fix is :

gconftool-2 --type bool --set /system/osso/af/mmc/mmc-corrupted false