Answer:
The shelf life of Spam, as indicated by the Hormel website, is "indefinate". It also states that spam will be safe to eat a lot longer than the Enjoy By date, although the flavor will decrease over time.
Any sealed(canned) container will remain safe to eat until a few things happen.
1) A break in the seal, through punctures or through corrosion of the metal.
2) Any kind of swelling of the container, or a outrush of gas upon opening.
The first is self-explanitory. The second is from bacteria build up in the container when it was sealed. Sometimes in the commercial canning industry cans get through that have not been sterilized properly. I'm not sure the exact number of these "tainted" cans that get through, but they are miniscule in comparison to the safe cans that make it through.
Also, remember that heat will do damage to canned foods. Keeping that can of spam in the pantry will allow it to keep much longer than it would in say the trunk of your car. It will still be safe to eat, and provide you with needed protein and sodium in an emergency, but it won't taste quite as good.
Hope this helps.