Thanks to you two for those valuable information.
In fact, the undocumented way to use URPO presented by helihyv did the trick and that is great, exactly what I wanted!
I didn't knew for "Maemo5 does not support asking password in graphical environment" and so let's go with the xterm way...
@exuvo: You're right, I should use a key, and in fact I do it for security matter in certain circumstances, but thinking about printing (not so often) from the N900, to a (random) machine with cups and a printer attached, I find it easier (quicker) with the askpass method.
Thanks to devs for this now much more useful tool, My vote for competition will go for it ;-)