As reported on Danwei on July 23, "BOCOG's Security Department head Liu Shaowu said that Beijing had already set up special demonstration zones for protesters to express themselves" (link).

Mirlin168 visited the three protest zones on August 11, and found them free of protesters.

Last week Xinhua published an article that explains why there are no protesters in the protest zones:

Spokesperson: Beijing authorities receive 77 demonstration applications since Aug. 1

Beijing authorities have received 77 applications for demonstrations since Aug. 1, a spokesperson with the municipal public security bureau said here on Monday.

These applications involved 149 people, including three persons from overseas.

Most of the applicants applied to protest in public for issues like labor disputes, medical disputes or inadequate welfares, the spokesperson said.

Seventy-four applications have been withdrawn so far, because the problems those applicants contended for were properly addressed by relevant authorities or departments through consultations, added the spokesperson.

Two other applications have been suspended because their procedures were incomplete, the spokesperson said. In one of such cases, for example, the applicant applied to take children to the demonstration, which is against China's law...

...The one remaining application has been vetoed by the public security authority, as it is in violation of China's law on demonstrations and protests, the spokesperson said without elaboration.

Update: The New York Times published an article about Gao Chuancai, one of the would-be protesters whose problems have been "properly addressed by the relevant authorities": Would-Be Protesters Detained in China.