3D acceleration via Novueau worked in the fresh installation as well as after
the update in another installation (AMD E-350 plus GeForce 9500GT).
System had on-board GPU enabled and I can hardly see a reason why this should
influence any dedicated GPU. The issue seems to be related to my hardware.
I will see if this is related to older GeForce cards if I update the system
of my daughter (NForce 2 motherboard with onboard GPU plus dedicated GeForce
4400.
At least for those with newer GeForce cards (like the 9500GT sold by Think
Penguin) I would consider updating to Trisquel 6.0 ok, while I am still
unshure about older NVidia cards.