Think about it. The above radiation symbol looks a little like an angel, or a wheel. It could be a holy symbol, or a mystical artifact. After several generations, its original intent could be lost.
Other symbols suffer the same problem. If you put a sign with a whole human body, and an arrow pointing to a skeleton, a future culture could easily interpret this in reverse and assume they've found an ancient, buried life elixir.
The challenge involves determining how to communicate with a culture that does not yet exist. Several ideas have been proposed, and redundancy has been encouraged. The current idea is to make burial sites look as foreboding as possible, as if the very land itself is sick and dissuades travelers from staying. Others wish to put warnings in as many languages as possible, and leave spaces so that future cultures may write warnings in their local languages. Other ideas include passing tales of the land into oral history — essentially building new mythologies to warn people of the sickness in the land.
Various ideas have been proposed regarding the last — some have suggested putting strangely-colored plants in the region, or plants with warning messages encoded in their DNA (there is an alphabet associated with the genetic code, so this is entirely feasible). Another idea involves inserting genetic code into cats (domesticated cats being considered more-or-less constant human companions) that causes them to change color in the presence of radiation. This trait will then be seeded into collective culture by use of myth and fairy tales (if a cat changes color, the land is sick and you should leave). Some have suggested an "atomic priesthood," which would tend to these sites and keep away interlopers. My personal favorite is simply placing a large concentration of human remains on the site so as to ward off any future seekers.
For obvious reasons, all markers must be large and lack value, lest they be moved by future civilizations.
Of course, many theorists suggest that marking the site in any fashion marks it as valuable (something any DM can appreciate), and that the wisest course of action is to avoid marking the site at all.
That certainly flows with the James Raggi mindset, anyway.
In addition to Hutchings' article at Teleleli, further information is available at Damn Interesting, Wikipedia, and Grist.
Gaming applications are obvious to anyone who plays D&D, Gamma World, or any similar sort of post-apocalyptic game. Dungeon delvers in particular are famous for ignoring blatant warnings of danger to go raiding tombs, and so would be perfect for this sort of thing.