Lifetimee is sometimes defined as useful product life. Some shady MFRs will word their guarantee to mean usable life (meaning you're SOL). These are pretty rare now, to my knowledge.
More commonly, lifetime is defined as lifetime, but replacement with a current rough equivalent.
I've never sent a video card back, but I did send a stick of "lifetime guarantee" PC133 back once. They didn't make that anymore, so they sent me a stick of PC2700 DDR. For my needs this was actually a useful upgrade. For some people this would be of less use.