Plastic can take a long time to decompose and, usually, oil based is rarely sustainable even when a second or tertiary use can be found – PET bottle may, for example, recycle with a second use in the fabrication of car bumpers.
The white blocks used to protect items in transits are often made from expanded polystyrene which, unless especially modified can take an extremely long time to decompose in the environment – it is very resistant to the normal biodegrading mechanism and, of course, being white, very visible and attractive to scavenging wildlife seeking food! Just think of the ratio: Time in environment/useful time for artefact for virtually inert packing materials, certainly extreme!
An amazing alternative – grow your own packing! The use of fungi mycélium (roots) to bind organic waste together and thus create a packing material that is totally sustainable and compostable, to replace blocks of expanded polystyrene, has been proposed. Bayer has a pilot plant operating, see: http://www.ted.com/talks/eben_bayer_are_mushrooms_the_new_plastic.html.
As an aside consider the sustainability golf balls between 100 and 1000 years in the environment http://edition.cnn.com/2009/SPORT/11/04/littering.golf.balls/index.html and; put that in perspective a thousand years ago we not only didn’t have the internet “the West” didn’t have printing with movable type – Caxton is fifteenth century!
However, with items that preserve so well in the environment the “lost” can be “found”… http://www.myfoxboston.com/dpp/news/special_reports/weird-job-golf-ball-diving-20100614
And he is not the only such person – there are many others following similar “careers”!