Apple has released an update to OS X that is specifically for the newly released iMac systems. The update addresses some graphical anomalies that occur in the new systems with the current version of OS X 10.6.4, and also fixes some compatibility issues with reading SDXC memory cards.
In addition to the 10.6.4 update, Apple has released a driver software package for the new Magic Trackpad, and is required for using the trackpad. In addition, it enables inertial scrolling and three-finger dragging gestures on the latest MacBook and MacBook Pro models.
The OS X 10.6.4 update is a 452.62MB standalone update from Apple's support site, but should also be available via Software Update.
The Trackpad driver update is available at Apple's support site as well (75MB). It is included in the 10.6.4 update for iMacs, so you should not have to install it if you have purchased a new iMac. For more information about the trackpad update, see this Apple knowledgebase article.