@ nicolai : Thank you for your help (I am sorry, I am a beginner in the use of a UNIX system : I didn't know that there was a kind of registry base in this system ).

I think I find the solution from your hint :

gconftool-2 --all-dir /system/osso/connectivity/IAP
=> List saved wireless networks with strings I don't understand (is there any logic for the string choice by the system or is it totally random ?)

gconftool-2 -a /system/osso/connectivity/IAP/String_I_dont_understand
=> Give all the variables in the directory (contain the real network name and its password without any encryption )

@ theonelaw : No, I just scare to loose this password (after I fight long time to have it...).