LOL -
I just posted a reply to the same kind of question. Let me re-cap...
For the most part, desktops will tend to be slightly faster than laptops, if anything, because desktops use larger, less energy-conscious drives. Whenever you tip the balance towards energy efficiency, you generally lose performance.
There really is little difference between the two machines, though. They use the same memory, the same processor, and neither need a high-end video card to do video editing - the integrated Intel video found on the standard Macbook is quite capable of keeping up with the video needs. In fact, my old crusty G3 iBook can keep up with the video signal from my gear, although I wouldn't try to edit with that laptop! (It will get the job done, but not before I start collecting Social Security). High-end video cards are for 3D model designers or people using Bootcamp to play games in Windows.
The only real difference is the hard drive. You can get larger, faster drives for the iMac, giving it a performance boost, but it's tethered to the wall. If you think you might go mobile, get the macbook. The Macbook has the ability to capture directly from firewire-capable cameras, and editing in the field is nice to be able to do.
If you're going to hook up a firewire external drive to use for editing, it makes very little difference which machine you get. On a side note, USB 2.0 is theoretically faster than Firewire, but Firewire's actual throughput and computer-less connectibility smokes USB, hands-down. USB, however, works in any machine.
Like I told the other user, you can't really make a "wrong" decision. Both machines are fully capable of editing video, but each offers its own bonuses. If you go with the Macbook, get the faster 7200rpm drive. It will run hotter and sap time off your battery's life, but it will give you better performance overall.
If you have money to burn and are serious about video editing, go with a mac pro desktop. It has much higher capacity for RAM and hard drives, and also comes with a true desktop cpu. Yes, the iMac uses the same laptop chip that the Macbooks use - that's how they keep their slim figure!
Good luck, and congrats on your purchase!