I don't see the demise of the cards themselves on store shelves for a while, but new camera models that use SM I think are dwindling. Manufacturers are going either with CF, which, with a Microdrive, can hold 12GB of photos, or, for the more beginner cameras, SD or another smaller card. The newer cards are faster and smaller. Also, SM could come in sizes up to 512MB, but I think that many SM-using devices only take up to 128.

They were great at the beginning, but I think that technology has moved on. But I'm sure you will be able to buy extra cards for quite a time into the future.