I agree dockwalliper. I dig everything out of wood chip play grounds. I feel better removing all junk metal out of there for the saftey of the kids, I've pulled some pretty nasty things out of playgrounds.
My worst problem with cents is that on my detector I can tell the difference between solid copper cents and copper coated zinc. If I know I'm going to be digging up a corroded post 1982 cent, it doesn't seem worth it to me. On the other hand, as every one has stated, they do all add up even if it is very slowly. I guess it's always better to get a corroded penny than a pulltab.
I'm starting to consider saving all the pull tabs and cashing them in for recycle value. It might end up paying off better than the pennies do.