Have you used the knife hard lately? Or put lateral pressure on the blade? I assume you are talking about horizontal blade play and not vertical? I have a couple riveted knives including a Salt 1 that has been in my pocket for a couple years being used as a money clip and haven't had a problem.
If you want to try doing it yourself instead of sending it to Spyderco (and possibly not getting the results you are looking for) there is something you can try. As a warning, anytime you do any work your self there is a chance something won't go right. But it shouldn't be that big of a deal to tighten it up a little. I would get a very small socket that just goes over the pin/ rivet and a hammer. Put the knife on a hard surface and place the socket over the pin and lightly tap with the hammer. After each tap open the blade and check to see how it feels. I would use really light taps until you see how the knife behaves. It would probably be better to do more lighter taps than one hard tap which might make it too tight. Unless the other rivets are just really loose I would only worry about the pivot. If you by chance get it too tight you can hold the handle (covered, in a vise is probably best) and put pressure on the blade near the handle to push the handle slabs apart with the blade tang, and again go slowly. I would be sure to cover the blade so you don't cut yourself. Some cardboard followed by tape would be a good start to making it a little more safe.