From the Horse's Mouth
Well, Mommas, it just so happens that my father is heavily involved with and gainfully employed in the Municipal Solid Waste Industry (and, no, he's not a mafioso!). So I did what any curious reader of this thread would do -- I called him and got the scoop.
First off, both the statements "nothing breaks down in a landfill" and "everything breaks down in a landfill" are both false. How much garbage can break down in a landfill is determined by the geology of the landfill, the construction of the landfill, and the compaction rate of the landfill. The greater the compaction and the more water-tight the landfill is, the less decomposition you will have. So if you really want to know about your trash, contact your local landfill administrator and ask him/her.
Dad also said that in order to promote carbon sequestration (offsetting the bad with the good), you'd be better off choosing paper bags over plastic since young trees absorb CO2 at a much faster rate than more mature trees. So it would be better, if you need a bag at all, to take one of paper than of plastic, the production of which is not nearly as environmentally friendly overall.
So, in short, don't use a bag unless you must. If you do, use a paper one. And in the meantime, do everything you can to reduce what you send to the landfill.
So I'm off to hang my head in shame over the plastic garbage bags we use at my house.

Does it help that I buy the Seventh Generation ones?
