Apple has released the iMac (Mid 2010) Display Brightness Update 1.0 (http://support.apple.com/kb/DL1080). It addresses an issue with 21.5-inch iMac (Mid 2010) display brightness.

It’s a 378KB download and requires Mac OS X 10.6.4. You can obtain it via the Software Updates component of Mac OS X’s Systems Preferences app.