While this thread may have started based on a question about an Al frame (gasp! -- not as bad a CF at least), the topic seems quite relevant to old steel frame pundits. I didn't see fatigue addressed at all in that Wiki link.
Stiffness, or modulus or elasticity, is a physical property of a material. Another physical property is melting point. The modulus can't change any more than the melting point, whether due to fatigue or other factors. The (tensile) strength of the material is a mechanical property, and can be manipulated. Steel like 531 has a higher tensile strength than cheap gas pipe steel, so less of the 531 steel can be used yet still result in a strong tube.
Racers used to speak of well-used steel frames getting soft -- I don't believe this to be true. I think it's a big placebo, and a route to a shiny new bike. If anyone can point to measured differences -- not anecdotal ramblings -- of a specific frame getting softer, I'd welcome looking at the numbers.
Hey, one more reason to keep riding your old steel frame.
Alan Cote Williston VT
Wikipedia has an a very article on different frame materials for bicycle frame and fatigue. suggest you read it and drawn your own concussions Charles Nighbor Walnut Creek