There are several phases to a revolution and the most significant one from the point of view of actual change afterwards, is if security forces join the revolutionaries and stop taking orders from the government. The reasons for this are obvious.
It might be the case in Iran now. According to this STRATFOR report I just received:
December 27, 2009
STRATFOR Iranian sources say that unconfirmed reports indicate as many as 16 people are believed to have been killed so far in the Dec. 27 protests in Tehran, including one Basij militiamen, and that several security forces have been injured along with rioters. There are also reports that there have been some desertions by security forces to the side of the protesters.