Apple has released a couple of security updates, for both Snow Leopard and Leopard systems. In addition, there are a couple of firmware updates available for the 27-inch iMac, which will update the system management controller and EFI firmware. These updates should be available through Software Update for applicable systems.
According to the support file for the security update, this update fixes a problem where maliciously crafted embedded fonts could lead to code execution when downloading or viewing files containing them. The firmware updates for the iMac address problems that caused high processor usage when playing audio through the headphone jack, where the display backlight would refuse to turn on after waking the iMac from sleep, and problems when running the iMac in "Target Display" mode (using the Mac as an external monitor).
The updates can be downloaded from the following locations (though right now they appear to be not accessible at Apple's site)
Security Update 2010-003 (Leopard-Server) -- 379.5MB
Security Update 2010-003 (Leopard-Client) -- 218.6MB
27-inch iMac EFI FW Update 1.0 -- 2.1MB
27-inch iMac SMC Firmware Update 1.0 -- 397KB
Be sure to back up before installing any of these updates. If you are installing the iMac firmware updates, we recommend minimizing any conflicts by disconnecting any peripheral devices you may have attached. Run the updates, and allow the system to fully install without interruption. You may see the system automatically reboot several times during this process, and it may take a number of minutes to install. Do not force the system to restart if it seems to take longer than usual.