What I'm saying is that if swapped in the N900's sensor, that EF 1200mm focal length lens would act as a 7200mm one, a frightening zoom (yes, you would have to have enough light and shutter speed to actually make use of it, but that's a different story).

As for the usefulness of digiscoping, it all depends on whether you know what you're doing. For example, if you're trying to make a photo of a planet, those monster Canon/Nikon lens (and DSLR image processing algos) fail miserably as they were not made to deal with astronomical imagery, no matter the depth of the wallet, a medium-class telescope and a cheap webcam will win every time at a fraction of a price of the big glass. Now, the cool part is that with fcamera is that we don't need that USB webcam, the wire-jungle, notebook, power banks - all that can be replaced elegantly with a single N900.