Here's a bit more context on emissions from waste-to-energy plants. According to Sweden, their plants produce 90-99% less air pollutants than they did in the 1980s. There's specific information about the drop off of each pollutant during that time and a really interesting graph showing the rise of electricity production and the decline in pollutants, in this document:
http://www.avfallsverige.se/fileadmin/uploads/forbranning_eng.pdfAnd FWIW, here's research the U.S. EPA did on the subject: "per unit electricity generated, greenhouse gases emissions from landfills are two to six times higher than those generated from plants that burn waste."
More here:
http://www.epa.gov/sciencematters/april2010/scinews_energy-from-waste.htm @cliffmeixner: It is a good question. Obviously, worst case senario is that the toxic ash is disposed of and seeps into groundwater. Again, here's what the EPA has to say about it (scroll down to "Municipal Solid Waste"):
http://www.epa.gov/cleanenergy/energy-and-you/affect/sw-generation.html 
