Obama's stance is obvious. He could care less. Annoying flies, more crazy people with their antiquated ideas of liberty and freedom. It's quite droll for Obama. He'd rather be on camera talking(maybe can run over Rush on Leno). :)
All joking aside I believe the important question is:

WHY are these people protesting? Answer: The belief in the threat of tyranny
from a one world government referred to as the New World Order. That's the central theme I've been hearing from the protester sites and videos.

While it is true that the protesters are from many different groups and some groups have higher priorities or different goals than others. It seems that the majority agree on this premise. They are worried about the replacement of the U.S. Dollar with a One World Currency. The Anti-New World Order stuff. The commonly held view is that the Central banks of the world control the world. And before they were in the shadows but now they are coming out into the open. Rumors of protesters getting arrested and the police seizing their cameras and backpacks and falsely proclaiming the items: "This is a gun, and the backpack is a bomb!"
Allegations that "The Pentagon is rebranding the low level protesters as terrorists" found from a DoD manual. And this morning's news of the arrest of infowars.com reporters who tried to talk to the army's PR rep and were informed later that they are now on the terrorist list by a laughing and mocking police officer just add more credence to this scary conspiracy theory that may have some truth to it. 
July 21st 2009 Video Clip of Ben Bernanke Congressional Testimony
while being questioned by Congressman Grayson.
Ben Bernanke on $550B of loans to foreigners (or 'central liquidity swaps' in Federal Reserve-ese').   Which financial institutions received this money? Bernanke's answer: I don't know. As the Fed was lending this money, the dollar increased by 30% in value. Grayson asks, was this a coincidence? Bernanke's answer: yes.

He doesn't have any answers and this furthers the conspiracy theory in doing so. At the very least you gotta think WTF?

h

The web site you linked to was formed by the Pittsburgh Organizing Group, a self described anarchist organization. They engage in facilitating a logistical core base for mass mobilizations and protests. They have done similar work at the 2002 anti-IMF "People's Strike" and the 2008 Republican Convention.

At this G-20 Summit, they are the umbrella group, just as other similar organizations have done so in other cities that G-20 Summits have been held, to provide structure and logistics to arriving protesters.

In general, they consist of a diverse group of protest organizations, ranging from the established, such as PETA and Greenpeace, to fringe groups like, Dykes on Bikes and Rad Queers.

Their modus operandi follows logistical preparation by the "host" umbrella organization, with planning taking place months in advance. The more established groups make use of "feeder groups" which are smaller local organizations to swell their numbers on arrival.

Each group of course has its own agenda, but they tend to share certain core principles. They are anti-capitalist, pro-environment, more often than not leftist organizations.

In Pittsburgh some of the specific groups protesting and their purpose are outlined below.

PETA, People for the Ethical Treatment of Animals, they are there to participate in some sort of nude sit in to promote their bare-it all anti-fur campaign.

Greenpeace is there to warn of the dangers posed to the planet by the greed of the developed world.

Dykes on Bikes has some sort of demonstration aimed at further exposure of the group and promotion of gay and lesbian issues.

Rad Queers also have a sizable demonstration with a similar aim.

The International Burmese Monks Organization is present, protesting, by chanting and praying, to advance their cause for Burmese freedom.

The Thomas Merton Center and the International Thomas Merton Society are holding large gatherings using the feeder group model, advancing their ideal of a just and peaceful world through contemplative thinking.

There is Oxfam America, dedicated to finding long-term solutions to poverty, hunger, and social injustice around the world.

You can get a better gauge of the mindset of the protester from these quotes of 2 typical protesters interviewed by the AP.

Joshua Nichols said, "I am here because I think the G-20 is in part an organization among others that leads to the subjugation of people all over the world," he said. "We need to stand up and say that we're going to put an end to this or it's going to get ugly."

Lisbet Rattenborg said, "I'm here to protest the idea that a few countries can determine the economy for the world, the economic path for the world."

As far as an official response from Obama or his administration regarding the protests, there is none. On the practical side this is understandable, they do not want to draw attention away from the key objectives and message of the summit. From a political perspective it is also understandable. It is not to wise to draw attention to protesters emanating from the far left. Note that in any reference to the protesters they are never referred to as far left protesters, not even as leftists, the term, extremist is the only one used.