Was waiting for my iPad and they brought the Apple TV....
Thought it was going to be next week for this.
Won't be able to try it out until tonight though as at work now.
If you don't want/need to use a remote with your iMac then you can disable the IR receiver in Settings->Security & Privacy, General tab, and check Disable the remote control infrared receiver at the bottom of the page.
If you still want/need to use a remote with your iMac then you'll need to pair the remotes with the respective devices. On your iMac click the Pair... button on the same settings page as above. On the Apple TV point the remote at the ATV and press and hold Menu and Right for 6 seconds.