After a lot (really a lot) of tries, every thing is back to "normal".
Situation :
* kernel 2.6.28-15 doesn't ha any display when psb drivers are installed
* kernel 2.6.28-15 has an "ugly" display when psb drivers are not installed
* kernel 2.6.28-14 works fine, when linux-backports-2.6.28-14 is installed, and psb-kernel-source "reconfigured". I guess there is still some random freezes.