"and publicly announcing this will help these gaps to stay unfiltered?"
It is in Iran's best interest to filter as little as possible. If you're a devout WoW player, they'd rather let you spend time on that, being oblivious and happy, than risk you being pissed off that you can't play. The most important thing for Iran's government to do is to try and make sure that no more people join the protests, and that those who have get discouraged by the hardship and return to their "comfortable" lives. They want people to return to "normal" even if it is just a sham because they can control the people that way. That requires people not paying attention to what the government is really doing, which requires giving people somewhere to "bury" their heads. The Internet is GREAT for that. I never found so many ways to waste my own time until I first opened that Mosaic browser one day...
What Iran's government has been doing with regard to filtering has been disturbingly effective. Yes, the protesters are getting together and communicating with each other, but there's no reliable sources of verifiable news. No reliable death count. No clear picture of what is happening. Citizen journalism is great, but it pales in comparison with what real news-gathering resources can do. So foreign governments are limited in their response, and that response is even more limited in the audience within Iran that can see it.
Don't discount the ability to keep information away from the militia men as well. The Iranian government is more dependent than ever on the blind faith of their security forces. They must be fed the party line, and be made to swallow it. You don't get that kind of obedience when those forces are allowed to think for themselves. So you deny them the ability to gather data to make up their own minds.
So yes, Iran is not blocking all possible methods of communication, but they're effective enough that they still may pull this off.
Information is power, and the information required to make your own decisions is the ultimate expression of that power.