I'm running with Ubuntu 12.04 64bit on Dell Latitude E6420. The inbuilt microphone is not working, and is not listed on the sound settings dialog.
According to lspci, the device is:
00:1b.0 Audio device: Intel Corporation 6 Series/C200 Series Chipset Family High Definition Audio Controller (rev 04)
I've also looked at alsamixer, and can see a "mic" and "dock mic" but fiddling with these and increasing to max amplification doesn't appear to change anything.
Note: I've raised a certification question on the same topic here: