doh, since you mentioned ldconfig looking for mesa libs before nvidia i checked out my /etc/ld.so.conf.d/
and found, it's my fault:
i had a conf file named steam.conf with
/usr/lib32
/usr/lib/i386-linux-gnu/mesa
/usr/lib/i386-linux-gnu/dri
/usr/lib/i386-linux-gnu/gallium-pipe
inside.
removing that file and running ldconfig again as root solved the problem
it was a PEBKAC issue :-(
Thank you for your support
Cheers Samuele
https://bugs.launchpad.net/ubuntu/+source/nvidia-prime/+bug/1307466/comments/11