There is still discussion about t410 and t510 backlights in a thread about Quadro FX 880Ms here: http://www.nvnews.net/vbulletin/showthread.php?t=145105
but all recent stuff is unrelated to this buzzing issue.
This issue persists with NVIDIA's 256.35 driver. Have any NVIDIA employees had access to a T510 to check this issue yet? Thank you for not yet enabling backlight control by default, the brightness values passed are still most likely incorrect!