Genuine Fractals is a great tool to prevent pixels from becoming visible on bitmaps that are printed at sizes far beyond what their base resolution will support. Since a transit ad or exhibit graphic isn't subject to close scrutiny, the results can be quite usable.
There's no substitute for resolution, however, for many images. As you well know, landscapes with lots of fine detail need a lot of pixels per inch to hold together on a high quality print. I look forward to the time your future Nikon SLR will provide 3600 x 5400 pixels (just under 20 megapixels) so you can make 12x18 prints with the same quality as you print now.
Since Sports Illustrated is printed at no more than 150 lpi dot screen (133 is more likely), a two page spread image isn't terribly demanding. The detail shown in two pages of SI won't be anything like that you would see in a publication like National Geographic, even when you factor in the different page sizes.