For years before the US went to war in Iraq, Iran and Iraq were at war, which kept either country from causing as much trouble to the rest of the world. The current war in Iraq removed this military threat from Iran, which has allowed Iran to become more of a threat to the US and Israel.
The war in Iraq also forced the US to take on Pakistan as an ally in the war on terror. As recent events in Mumbai have shown, Pakistan has not proven much of an ally in this regard, and US concessions to Pakistan have also hurt its relationship with India.
Perhaps both of these consequences of the war in Iraq (just a couple of many) are worth it since we captured Saddam Hussein (who was only peripherally connected to the 9/11 attacks, if at all) and may succeed in bringing democracy there. There are many dictators in the world and many more countries where democracy doesn’t exist (Saudi Arabia, another ally in the war on terror, is an example of the latter), and the US can’t save them all.
aussieboy, did you write that the US has captured Osama bin Laden? I must not be following the news that well, since I didn’t know about that.