Is a bug , it will fixed in the next patch - maybe today will be the launch .
You can change it in the ini files . Information about the change - there are some problems if you change other keys in the configurator - up and down disappear - or if you change the lenguage of the game, the game reset all key mapping .
http://www.gog.com/en/forum/the_witcher_2/keyboard_remapping_issues/page4 Final tutorial:
1. Open Documents\Witcher 2\config (not instalation folder)
2. File DIMapping.ini contains keys that game knows (but not all will work on your keyboard) - use it as source
3. File user.ini contains your binding. You have to replace IK_SomeOriginalKey= with your key
4. You can delete all IK_PAD_* mappings, they are for gamepad
5. You can remap I Inventory key, M Map key...
6. You can verify your binding by opening Options from launcher - but it will break Up/Down arrows mapping. You have to delete added lines:
IK_Down=(GameKey="GI_Down",Value=1)
IK_Up=(GameKey="GI_Up",Value=1)
These keys are valid - you will control movement by cursor keys:
IK_Down=(Gamekey=GI_AxisLeftY,Value=-1.000000)
IK_Up=(Gamekey=GI_AxisLeftY,Value=1.000000)
IK_Left=(Gamekey=GI_AxisLeftX,Value=-1.000000)
IK_Down=(Gamekey=GI_AxisLeftY,Value=-1.000000)