Gnome 3 always on fallback upon login

I have a Debian Wheezy install, with Gnome 3 as the windows manager. I also have a NVidia video card for what it matters.

I autolog in gnome upon boot to my user account, but it always start gnome in fallback mode. After I logout and relog again in the same account (without changing options in the login windows), then it starts gnome 3 properly.

I was wondering if this could be linked to 3D rendering, but:

|> cat /etc/X11/xorg.conf

[...]

Section "Device"
    Identifier     "Device0"
    Driver         "nvidia"
    VendorName     "NVIDIA Corporation"
EndSection

Section "Screen"
    Identifier     "Screen0"
    Device         "Device0"
    Monitor        "Monitor0"
    DefaultDepth    24
    SubSection     "Display"
        Depth       24
    EndSubSection
EndSection

and

glxinfo | grep -e "direct " -e" renderer"

direct rendering: Yes 
OpenGL renderer string: NVS 4200M/PCIe/SSE2