You hope it shows up early, as it did for me. Mine was schedule to arrive anytime from the 28-30. It showed up on the 24th. I was very pleasantly surprised.
I would recommend that you get a hold of two 2GB modules to install in your machine. I hope you didn't get the expensive Apple RAM upgrade, but if you did, it's already decided.
If you buy the RAM separate, you save money (it's cheaper than the upgrade price on their website), AND you get to keep the original 1GB modules that come in the system. So, you could use that 1GBs in a different system, sell them, give 'em to a friend, or experiment on your dog. Whatever you want.
Find a great wallpaper that will fit your new huge iMac screen (huge to me, cause I came from using an iBook/MacBook to an iMac). The screen on this baby makes me feel like a small man

.
Also, rearrange your desk/computer room in preparation for the machine. If you have been using laptops prior to this iMac, get your laptop away from the desk and use it like it's a real laptop... in your lap on the sofa! Prepare that desk by buying new fancy lamps, cleaning, rearranging items, etc.
Also, read every little bit of propaga... I mean information on the Apple website about your new upcoming machine. Go on GeekBench and compare iMac scores to your current machine.
Refuse to consume anything with processed sugar until the iMac arrives. Do whatever you want, man!