Expressing support and best wishes for the protestors gives them a boost in spirit that they need if they are to succeed.
I think its pretty insulting to the Iranian opposition, especially given the "spirit" they have demonstrated thus far, to suggest that their morale will crack if they aren't given an explicit and direct endorsement by a foreign leader, particularly the leader of a country that has pointed to their nation as an enemy for decades.
Not so. A lot of the Soviet dissident leaders took great pride in knowing that Reagan fully supported them. It gave them more hope and willingness to keep up the fight knowing that someone outside the USSR was supporting them. And we did look at the USSR as an enemy for decades (rightfully so).
Same thing with Poland the Solidarity movement. Soviets cracked down, Reagan full-heartedly supported Solidarity and the Polish people, telling the current leadership that to continue the crackdown would mean an end to "business as usual" with the US.
It's not that Obama is afraid to "take a hit" for supporting the Iranian opposition, I just don't believe that he wants them to succeed. I honestly don't believe that he believes in freedom and democracy enough to come out in support of the opposition. He wants business as usual with Iran and Ahmadinajand (sp?). A freedom and democracy uprising basically undermines his Cairo Speech To The Muslim World. I think he's too full of himself to support the opposition in Iran.