I think there are two big reasons fewer people believe.

1) It's hard to see past one's own back yard, and in some areas it has been unseasonably cold.

2) Due to the recent cold some global warming sceptics are using this as a chance to advance thier theories.

The attached dives into this topic very nicely.

"Have you heard that the world is now cooling instead of warming? You may have seen some news reports on the Internet or heard about it from a provocative new book. Only one problem: It's not true, according to an analysis of the numbers done by several independent statisticians for The Associated Press."

"The case that the Earth might be cooling partly stems from recent weather. Last year was cooler than previous years. It's been a while since the super-hot years of 1998 and 2005. So is this a longer climate trend or just weather's normal ups and downs?"

http://news.yahoo.com/s/ap/20091027/ap_on_sc/us_sci_global_cooling

Nina do you have a link for the tudy you mention?
Denial is too strong, 56% is still a majority.

A current poll states that al gore is the biggest environmental hypocrite.
http://www.globalwarming.org/2009/10/27/deconstructing-global-warming-presentation-by-dr-richard-s-lindzen/

but only about 450 people responded.

i think people are becoming better informed about the causes of climate change.
I noticed that many scientists disagree about the extent of global warming and its causes. This could be a reason why the study you mentioned showed a decline. Check out the link below, it's a powerpoint presentation available online by MIT Metereology prof.
http://www.globalwarming.org/2009/10/27/deconstructing-global-warming-presentation-by-dr-richard-s-lindzen/

The reason that I do not trust figures related to the public perception on global warming be it pro or con, is that they are usually slanted by the side conducting the study.

Personally, I think that global warming proponents vested way too much too early on in their theories. If they were wrong, then great, as a person that lives in a developed country, thank goodness we cut it off at the pass before we spent zillions on wild suspicions.

If, however, they were right, then how irresponsible of them to have urged along a scientific process on the basis of faith, discrediting themselves among the populace early on, losing their support when it would of had been most needed.

To me, that is perhaps why the initial global warming band wagon has receded somewhat. When science is presented with scare tactics, or the people are told to trust blindly, as if though it were a faith based religion, the net result is just what you may be seeing now. If a series of scenarios are blindly circulated, that this will happen by 1995, and this by 2000 and this by 2005, and then they don't happen, in the eyes of the public it becomes a discredited movement.

There are several reasons that could account for this.

Recently, many high profile figures in the scientific community have recently spoken out against doomsday predictions built on what many feel to be inaccurate models. Examples include

1) this letter to the editor-in-chief of the American Chemical Society castigating his recent strong remarks in favor of current global warming models: http://pubs.acs.org/cen/letters/87/8730letters.html

2) a recent petition published in Nature magazine for a revised assessment of climate change: http://www.nature.com/nature/journal/v460/n7254/full/460457b.html

3) an open letter by 54 physicists to the American Physical Society: http://striky.ece.jhu.edu/~sasha/Public/APS.open.letter.09.pdf

4) the work of Prof. Lindzen of MIT, whose recent work indicates that not only do the UN-accepted models of climate change poorly fit the data, they are nowhere close to accurately describing it: http://www.examiner.com/examiner/x-7715-Portland-Civil-Rights-Examiner~y2009m8d18-Carbon-Dioxide-irrelevant-in-climate-debate-says-MIT-Scientist

Couple all this with
a) the last winter being one of record cold for many people in the country

b) the Farmer's Almanac predicting the same for the upcoming winter: http://www.usatoday.com/weather/news/2008-09-09-farmers-almanac_N.htm

c) recent stories about how sunspot activity has an impact on global temperatures: http://www.newscientist.com/article/dn11650-climate-myths-global-warming-is-down-to-the-sun-not-humans.html

d) throw in what many perceive as absurdity in events such as the recent book whose thesis is that your dog is worse for the environment than an SUV http://www.amazon.co.uk/Time-Eat-Dog-Sustainable-Living/dp/0500287902 and http://www.stuff.co.nz/dominion-post/national/2987821/Save-the-planet-eat-a-dog

d) British Prime Minister Gordon Brown's recent proclamation that we have only 50 days to save the planet http://news.bbc.co.uk/2/hi/uk_news/8313672.stm, which to many reeks of a pushy car saleman trying to close the deal

e) sit down for a chat with someone who remembers how in the 1970s it was supposed to be global cooling that would destroy us all

f) or, finally, contemplate the fact that the Climate Research Unit (the agency responsible for storing global temperatures data) now admits that it has lost or destroyed all of it's raw data (http://www.theregister.co.uk/2009/08/13/cru_missing/)

.... and you have the makings of serious skepticism in the public regarding global warming predictions.