Bartertown.com

What does this radioactive symbol icon mean next to a PM?
http://i.imgur.com/WMRqEwX.jpg

It means you changed the icon to that symbol inadvertently.

When you write a PM, you can select to include a symbol next the the header. So the person selected the radioactive symbol.

It doesn't actually mean anything unless you've been recently bitten by spiders or have developed unusual powers.