Take the "life span" of inks as advertised by the manufacturers with a grain of salt, much like the shiny, new, 3Gb/s ratings of SATA II or "look! 480Mbps with USB2!" In real life, you won't get that. The 20, 80, or 100 years they quote is based on a set of rather stringent lab-quality requirements: the print has to be unexposed to air (i.e., framed), in a range of temperature, not exposed to heat or direct sunlight, etc.
Someone posted upthread that glossy prints last longer than matte prints. That's interesting--it's the opposite of what I've heard about print longevity. I'll see if I can find some more conclusive data and post here.