How do I make my external Thunderbolt 27" display's brightness match my iMac's brightness? I'm running my 27" iMac with a 27" Thunderbolt display, and I'm having trouble getting their brightnesses to match. It seems like checking "Automatically adjust brightness" on both causes different results.

What's the best way to get these two displays to always have the same brightness?
The best, albeit somewhat expensive, way is to use a spectrophotometer to create accurate color profiles for each of the two monitors. Short of that, you can use the built-in calibration software, accessed using the Monitors control panel, to create the profiles by eye.

According to Apple's documentation on Calibrating displays with built-in ambient light sensing you should disable the automatic feature while creating the color profile. They claim you can then re-enable it, but for color critical work I would consider leaving this function disabled and controling the environmental light instead.

Unfortunately, there are reports of Mac OS X 10.7 not working properly using different profiles for two monitors. You may have to manually (or use an Automator script to) assign the correct profile to each monitor after every restart.