Has your new 21.5-inch iMac been having display brightness issues? Well, then, you may want to check out the iMac (Mid 2010) Display Brightness Update 1.0 from Apple, released Thursday afternoon US time.
The update "addresses an issue" with the new 21.5-inch iMacs in regards to display brightness. Unfortunately, Apple gives no further explanation as to what specifically the update addresses, but some users have been complaining of buzzing screens on the 21.5-inch iMac models since last year.
The update is available on Apple's Website or via Software Update, and requires OS X10.6.4 or later.