I see some unused spaces in qwerty variant. Adding more letters/symbols could be optimal. For example if you add: ñ ç € º ª ¿ ¡ ' (this last one is the acute acent, used for compose: á é í ó ú) keyboard would be valid for spanish too. In a normal qwerty keyboard and in N900 spanish variant ñ is on the right key of l, and ç on the right key of ñ.

I don't see other usual symbols as: # £



Edited for correction: I didn't see the "_" symbol, it's over the "p". "-" is the key on right to "0".