With my XT and various PowerShots, my backup strategy involved a 500 GB external drive and burning DVD's. With a 60D and 7D, I've had to up the capacity quite a bit. Now it's two 1.5 TB external drives and burning BluRays (25 GB per disc).
On the external drives, which I manually mirror, I keep straight-out-of-the camera images in one directory tree and post processed images in another. I can shoot anywhere from 500-2,000 images per event. Admittedly, when lighting is reliably even (particularly in color), I shoot JPG. But, as things get moe challenging light-wise, more likely to need post work, or more important, I'll shoot RAW. 25 MB RAW images add up in a hurry. Those 1.5 TB drives have nearly 1 TB of photos.
So, as I contemplate another set of 1-2 TB drives, how do you store and archive your images?