In all those cases a program like Genuine Fractals is used to resize the pictures to up to 10x their actual size... This is what all agencies use when they need files of this size ...
Dumb question: If you re-size an image with Genuine Fractals, how good is the end result? How much better are the results than, say, using CS5 to interpolate an image? Most my work is for editorial clients, in which case my 5DIII's resolution is plenty for two-page spreads at 300 dpi. Even my 5DC was adequate in this regard.
Genuine Fractals claims that you can enlarge an image by 1000% without sacrificing quality, which just sounds insane to me. Is this legit claim, or just marketing hype? I've got some 30x20 prints taken with a 5DC, which is about 2x it's resolution at 300 dpi, but they look pretty good to me. I'm just curious how much better the results might be with Genuine Fractals, and heck, the software is pretty cheap.
You can find reviews of the various resampling methods without too much trouble, but it basically boils down to this:
Regardless of what method you use, view the image at actual size (hold a ruler to the screen and adjust the magnification until the onscreen ruler matches) and do your final sharpening that way before sending it to the printer.
If the final resolution is at least 100 ppi, just print it. Chances are excellent your print driver will do a better job at resampling it than anything else you have.
If the final resolution is at least 85 ppi, make a test print. Chances are still good that the print driver is your best bet. If you don't like the results, resample in Photoshop to somewhere in the area of 150 ppi. Try each of the bicubic options and go with whichever looks best. Be sure to sharpen again after resampling -- and, of course, revert to the unsharpened version before doing the upsampling.
If the final resolution is even less than that, you're hopefully making a billboard or a mural. Hand the best, NOT resampled image you've got to the printing agency and let them worry about it -- that's what you're paying them for.
If you're the printing agency, buy every tool there is on the market, learn which is best suited for which types of images, and how that fits into your workflow.
Note that, with modern DSLRs, you either have to crop insanely or be printing something the size of a door before you start to run into these resolution guidelines. If you're shooting with something other than a modern DSLR, then high-resolution printing obviously isn't a concern.