had the same issue with the same configuration - t420, win 64bit, had the lenovo-suggested driver installed (and re-reinstalled) and camera wasn't working, with same behavior as described in your issue and picture.
for my case, an addition to the behavior described is: when i had the communication settings screen open (i.e. where the camera should appear but didn't), the camera's green Led was lit, suggesting it's started.
The integrated camera does NOT appear to be supported in Windows 8. Look at the driver page for the Lenovo T420. You will notice that there are Windows 8 drivers for many devices. Under the camera and card reader section, Windows 7 (32-bit, 64-bit), Vista (32-bit, 64-bit), and XP are listed. Windows 8 is not.
Lenovo Thinkpad T420 Drivers Windows 7 (32-bit Download)
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