I tried one more time with FGLRX :

1/ Removed radeon driver (sudo apt-get remove --purge (...)-ati (...)-radeon)
2/ Installed Catalyst 10.5 with ati installer script
3/ Deleted xorg.conf (after backup)
4/ Ran : sudo aticonfig -f --initial=dual-head --adapter=all
5/ Modified my kdmrc file disabling multiseat mode back to single "normal" mode (launching a single Xorg without any "exotic" parameter)
6/ Restart
...
7/ Same system crash :-(

Empty Xorg.log file !

I've found this in the kern.log file :

May 30 14:41:37 belgarion kernel: [ 9.389229] fb1: radeondrmfb frame buffer device
May 30 14:41:37 belgarion kernel: [ 9.389232] [drm] Initialized radeon 2.0.0 20080528 for 0000:03:00.0 on minor 1
May 30 14:41:37 belgarion kernel: [ 9.390411] vga16fb: initializing
May 30 14:41:37 belgarion kernel: [ 9.390413] vga16fb: mapped to 0xffff8800000a0000
May 30 14:41:37 belgarion kernel: [ 9.390415] vga16fb: not registering due to another framebuffer present
May 30 14:41:37 belgarion kernel: [ 9.411898] Console: switching to colour frame buffer device 210x65
May 30 14:41:37 belgarion kernel: [ 9.476605] [drm] 2: conflicting encoders switching off 9-pin DIN-1
May 30 14:41:37 belgarion kernel: [ 9.476607] [drm] in favor of DVI-I-1
May 30 14:41:37 belgarion kernel: [ 9.486597] [drm] 2: conflicting encoders switching off 9-pin DIN-1
May 30 14:41:37 belgarion kernel: [ 9.486599] [drm] in favor of DVI-I-1
May 30 14:41:37 belgarion kernel: [ 9.877968] type=1505 audit(1275223297.234:5): operation="profile_replace" pid=1147 name="/sbin/dhclient3"
May 30 14:41:37 belgarion kernel: [ 9.878342] type=1505 audit(1275223297.234:6): operation="profile_replace" pid=1147 name="/usr/lib/NetworkManager/nm-dhcp-client.action"
May 30 14:41:37 belgarion kernel: [ 9.878546] type=1505 audit(1275223297.234:7): operation="profile_replace" pid=1147 name="/usr/lib/connman/scripts/dhclient-script"
May 30 14:41:37 belgarion kernel: [ 9.880119] type=1505 audit(1275223297.234:8): operation="profile_load" pid=1151 name="/usr/lib/cups/backend/cups-pdf"
May 30 14:41:37 belgarion kernel: [ 9.880568] type=1505 audit(1275223297.234:9): operation="profile_load" pid=1151 name="/usr/sbin/cupsd"
May 30 14:41:37 belgarion kernel: [ 9.881836] type=1505 audit(1275223297.234:10): operation="profile_load" pid=1152 name="/usr/sbin/mysqld-akonadi"
May 30 14:41:37 belgarion kernel: [ 9.882824] type=1505 audit(1275223297.234:11): operation="profile_load" pid=1153 name="/usr/sbin/tcpdump"
May 30 14:41:37 belgarion kernel: [ 10.014079] vgaarb: device changed decodes: PCI:0000:03:00.0,olddecodes=io+mem,decodes=nonewns=none
May 30 14:41:37 belgarion kernel: [ 10.014083] vgaarb: device changed decodes: PCI:0000:02:00.0,olddecodes=io+mem,decodes=nonewns=io+mem
May 30 14:41:37 belgarion kernel: [ 10.118624] [drm:drm_mode_getfb] *ERROR* invalid framebuffer id



I think i need help...