"I think the poll's finding that 38 percent of the public would favor a human mission to Mars is actually very encouraging under the current circumstances. After all, that number comes in the absence of any type of political leadership on the subject or any great promotional campaign to push the idea. If and when they do have those factors on their side, does anyone really doubt that they could get those poll numbers over fifty percent very quickly. "

I agree that if 38% of the public were in favor of Manned Mars, that would be encouraging.
I don't agree that a "great promotional campaign" would improve this support nor do I think "favor" translates to "support". With a Manned Mars, one must ask the "when" question. Can NASA sell the idea that it's budget needs to be increased in order to pay for it's preparation for a Manned Mars and what does this preparation actually entail. I would say that NASA is already "preparing" and it's "learned" that it's NOT a good idea to associate this preparation with a Manned Mars mission. The reason for this is that NASA has already spent billions already "planning" for Manned Mars and had little to show for all this money. Perhaps, after some time has past and some of the sting has gone, they can once again try another "great promotional campaign" route, but that doesn't mean they are actually going to do a Manned Mars, it's just a way to get more money to NASA. If "someone" can get a billion or two added to NASA's budget per year, then there is no doubt the NASA will gush endlessly about sending humans to Mars, but real progress would doubtful.
The preparation that NASA is currently are mainly ISS and the nuclear program- these are arguably "real" and "substantive" steps towards a Manned Mars- everything else NASA has spent money on regarding Manned Mars has been mostly studies and viewgraph presentations.
Now, can anyone realistically show how NASA is going to actually send humans to Mars, when at same time NASA is dumping money into ISS and the Shuttle program?
Now, some people who favor Manned Mars see ISS and Shuttle as a waste of "their" Manned Mars money. They simplistically assume that they can "get" the 5 billion per year from these programs and shift them over to a Manned Mars. What they fail to realize is the ISS is "their" Manned Mars program. And that it's not simply a matter of moving money around on paper.
To make this simple. I would say that unless "something happens" NASA will continue doing ISS for next 10 years. It will be pointless and huge waste of money- but that shouldn't stop a govt agency. After this NASA might start doing something as far as going sending Humans to Mars (more doom of wasted decades, changed plans, not enough money going to NASA [even though it's budget by this time could exceed 20 billion per year]).
Anyways, "something will happen". It's concievable that after 10 years the Shuttle or [Shuttle II] is still limping along. And that NASA is still trying to get more 3 crew living on ISS. But the world is changing.
Europe, China, and even India or other national players are going to do something and this something will influence what evenually NASA does. Next [and more important] you have what's happenning outside the realm of the govt. There is probably even chances that a sub-orbital spacecraft win the X-prize, before the next Shuttle is launched. We might be in a similar transition of the computer industry- when mainframes were the only computer, and this thing called a micro-computer entered to stage.