Marco, I was in the area for three weeks in January 2002, but not on motorbike. I travelled from Agadez to Timia in a loop circuit by camel. The road from the south to Timia is rough, particularly near the top of the rise near Timia, but I would think quite passable by motorbike.
I didn't go as far north as Iferouane, or to Arlit. (I headed south again through the area near Mt Bagzane) At the time it was necessary for travellers to obtain approval for travel north of Adadez, from the police, and to have a guide. -They like to know who is travelling where, in view of the troubles in the past few years. It is a great area, and well worth a visit. Don't forget to try the grapefruit and oranges in Timia -fantastic !!