It all started when I installed nvidia's own (download from website) driver. Uninstalling it and reverting back to nvidia-current didn't solve the problem, so I have been playing with this for a while. It seems glx support is missing, and my intel graphics is not responding. gnome loads only in fallback mode.

Here are some outputs:

glxinfo
name of display: :0.0
Error: couldn't find RGB GLX visual or fbconfig 

glxgears 
Error: couldn't get an RGB, Double-buffered visual

optirun glxgears
works fine

lspci | grep VGA
00:02.0 VGA compatible controller: Intel Corporation 2nd Generation Core Processor      Family Integrated Graphics Controller (rev 09)
01:00.0 VGA compatible controller: NVIDIA Corporation GF108 [GeForce GT 540M] (rev ff)

Here's the content of log file when just after running glxinfo:

less /var/log/Xorg.0.log | grep gl

[   112.156] (II) LoadModule: "glx"
[   112.157] (II) Loading /usr/lib/xorg/modules/extensions/libglx.so
[   112.157] (II) Module glx: vendor="X.Org Foundation"
[   112.157] (II) UnloadModule: "glx"
[   112.157] (II) Unloading glx
[   112.157] (EE) Failed to load module "glx" (module requirement mismatch, 0)

Some more info...

lsmod
Module                  Size  Used by

bbswitch               13612  0 
pci_stub               12623  1 
vboxpci                23195  0 
vboxnetadp             25671  0 
vboxnetflt             23480  0 
vboxdrv               320372  3 vboxpci,vboxnetadp,vboxnetflt
parport_pc             32689  0 
ppdev                  17074  0 
bnep                   18141  2 
rfcomm                 46620  12 
binfmt_misc            17501  1 
snd_hda_codec_hdmi     32049  1 
snd_hda_codec_realtek    78147  1 
joydev                 17458  0 
uvcvideo               76750  0 
videobuf2_core         32852  1 uvcvideo
videodev              120310  2 uvcvideo,videobuf2_core
videobuf2_vmalloc      12861  1 uvcvideo
videobuf2_memops       13405  1 videobuf2_vmalloc
snd_hda_intel          33492  3 
coretemp               13401  0 
kvm_intel             132760  0 
arc4                   12530  2 
snd_hda_codec         134213  3 snd_hda_codec_hdmi,snd_hda_codec_realtek,snd_hda_intel
kvm                   414111  1 kvm_intel
iwlwifi               386837  0 
i915                  520799  2 
snd_hwdep              17699  1 snd_hda_codec
snd_pcm                96668  3 snd_hda_codec_hdmi,snd_hda_intel,snd_hda_codec
psmouse               100389  0 
drm_kms_helper         49113  1 i915
ghash_clmulni_intel    13221  0 
snd_seq_midi           13325  0 
snd_rawmidi            30513  1 snd_seq_midi
btusb                  22475  0 
drm                   288436  3 i915,drm_kms_helper
serio_raw              13216  0 
snd_seq_midi_event     14900  1 snd_seq_midi
snd_seq                61555  2 snd_seq_midi,snd_seq_midi_event
mac80211              535936  1 iwlwifi
bluetooth             209249  22 bnep,rfcomm,btusb
snd_timer              29426  2 snd_pcm,snd_seq
snd_seq_device         14498  3 snd_seq_midi,snd_rawmidi,snd_seq
snd                    78921  16 snd_hda_codec_hdmi,snd_hda_codec_realtek,snd_hda_intel,snd_hda_codec,snd_hwdep,snd_pcm,snd_rawmidi,snd_seq,snd_timer,snd_seq_device
cfg80211              206797  2 iwlwifi,mac80211
aesni_intel            51038  1 
cryptd                 20404  2 ghash_clmulni_intel,aesni_intel
aes_x86_64             17256  1 aesni_intel
dell_wmi               12682  0 
sparse_keymap          13891  1 dell_wmi
dcdbas                 14439  0 
i2c_algo_bit           13414  1 i915
microcode              22804  0 
lpc_ich                17062  0 
soundcore              15048  1 snd
snd_page_alloc         18485  2 snd_hda_intel,snd_pcm
video                  19391  1 i915
mac_hid                13206  0 
wmi                    19071  1 dell_wmi
mei                    40691  0 
lp                     17760  0 
parport                46346  3 parport_pc,ppdev,lp
ahci                   25721  3 
libahci                31192  1 ahci
atl1c                  41102  0 

how can I fix this? any ideas?

Here is another thing I've tried:

sudo apt-get purge nvidia*
sudo reboot
sudo apt-get install bumblebee bumblebee-nvidia

didn't make any difference.

The most relevant post I found on the web was this: http://ubuntuforums.org/archive/index.php/t-1722306.html Here it's explained that the problem is with the priority of shared libraries that are loaded for glxinfo. Here's what I get when I look up the libraries:

linux-vdso.so.1 =>  (0x00007fff6bf8b000)
libGL.so.1 => /usr/lib/x86_64-linux-gnu/mesa/libGL.so.1 (0x00007f22d6ccd000)
libX11.so.6 => /usr/lib/x86_64-linux-gnu/libX11.so.6 (0x00007f22d6993000)
libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f22d65d3000)
libglapi.so.0 => /usr/lib/x86_64-linux-gnu/libglapi.so.0 (0x00007f22d63ad000)
libXext.so.6 => /usr/lib/x86_64-linux-gnu/libXext.so.6 (0x00007f22d619b000)
libXdamage.so.1 => /usr/lib/x86_64-linux-gnu/libXdamage.so.1 (0x00007f22d5f97000)
libXfixes.so.3 => /usr/lib/x86_64-linux-gnu/libXfixes.so.3 (0x00007f22d5d91000)
libX11-xcb.so.1 => /usr/lib/x86_64-linux-gnu/libX11-xcb.so.1 (0x00007f22d5b8f000)
libxcb-glx.so.0 => /usr/lib/x86_64-linux-gnu/libxcb-glx.so.0 (0x00007f22d5977000)
libxcb.so.1 => /usr/lib/x86_64-linux-gnu/libxcb.so.1 (0x00007f22d5759000)
libXxf86vm.so.1 => /usr/lib/x86_64-linux-gnu/libXxf86vm.so.1 (0x00007f22d5553000)
libdrm.so.2 => /usr/lib/x86_64-linux-gnu/libdrm.so.2 (0x00007f22d5346000)
libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f22d5129000)
libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007f22d4f25000)
/lib64/ld-linux-x86-64.so.2 (0x00007f22d6f54000)
libXau.so.6 => /usr/lib/x86_64-linux-gnu/libXau.so.6 (0x00007f22d4d20000)
libXdmcp.so.6 => /usr/lib/x86_64-linux-gnu/libXdmcp.so.6 (0x00007f22d4b1a000)
librt.so.1 => /lib/x86_64-linux-gnu/librt.so.1 (0x00007f22d4911000)

It seems "mesa" instead of nvidia-current is being used. However I didn't find any obvious link to /lib/nvidia-current in /etc/ld.so.conf (where the directories containing the shared libraries are listed). I don't know if there's anything missing or if this is normal.

thanks,

UPDATE: The problem was solved by updating to Ubuntu 13.04. It seems bumblebee was to blame, but I'm not sure what was going wrong...