Is deicing fluid harmful to the environment?
The main component of deicing fluid is either Ethylene Glycol (EG, toxic) or Propylene Glycol (PG, non-toxic) (source) and depending on the category applied different amounts are used annually. 

There have been studies of the environmental impact of deicing fluid, which basically states:


In water, EG is not persistent and biodegrades aerobically and anerobically.
In air, EG is not readily volatile and its atmospheric half-life is approximately 1 day
In soil EG is also not persistent and biodegrades anerobically in about 1 day
EG is disposed of and treated at standard wastewater plants


(I think the above comments also apply to PG, but somebody will have to confirm that since I can't find anything that mentions it).

PG is non-toxic and is used in quite a few products that you use every day, from pharmaceuticals to food additives and moisturizers. I can't confirm but one of the linked articles says that most of the DI fluid used in the US is PG based, which I would believe, but can't source.

And from the article quoting an EPA Report 821-R-00-0016, January 2000, Revised August 2000, 


  Although EPA does not use such a system, the
  U.S. Fish and Wildlife Service Classification
  System for Acute Exposures defines ‘relatively
  harmless’ as any chemical with an LC50 above
  1,000 mg/L. The test results … indicate that
  ethylene glycol and propylene glycol may be
  classified as ‘relatively harmless,’as defined by
  the U.S. Fish and Wildlife Service.


There is also one manufacturer using 1,3-propanediol which is a by-product of corn fermentation and should be relatively minor as far as environmental impact.

There is also considerable movement in airports to "go green" and recover used deicing fluid, especially considering that a larger aircraft can take more than 1000 gallons of diluted deicing solution at the rate of up to $12/gallon, quite a bit of which runs off the aircraft and onto the ground. In fact, the EPA has mandated that new airports in cold climates recover 60% of DI fluid.


Edit

It may be important to note that the point of DI fluid is to keep the snow/ice off the aircraft until it leaves the ground, not assist in flight into known ice. Depending on the fluid applied, they usually only stay on until the aircraft reaches about 100kts. This means that most of the applied DI fluid is going to end up in the airports wastewater system because most will run off the aircraft on the ramp/apron, not spread around the countryside as the aircraft flies around.