Also on V8Beast's post below, personally I don't see a significant difference in "big picture" like for like comparisons between crop and FF. Yes, the differences are there at pixel level, but who looks at pixels?
It really depends what you shoot and what purpose you shoot it for. For editorial work, how the photographer envisions a story layout while looking through the viewfinder - and therefore how he composes a shot - and how the art director actually lays out the image, are often very different. As such, it's not uncommon for an image to be cropped substantially. For instance, I might compose an image to be used as a two-page spread, and leave lots of dead space around the subject for text. If the art director chooses not to use the image how I envisioned, and heavily crops into the image, then the limitations of the file become much more exaggerated.
Even so, this isn't something most people stepping up from a point-and-shoot to a Rebel/xxD need to worry about, so why pay a premium for a FF sensor? Furthermore, certain forms of photography are more demanding than others in terms of dynamic range and high ISO requirements, so it's impossible to make blanket statements like "FF sensors are always superior to 1.6:1 sensors." Most images people take these days just sit on a hard drive, and only geeks like us pixel peep, so again, Canon would have it's work cut out for it to convince people to sacrifice megapixels for sensor size.