The short answer to this is that women can be imams if the congregation is female. In fact if two or more women pray together, one of them will have to be the imam, the same as if two or more men (or men and women) pray together one of the men among them will have to be the imam.
Imams in Islam are often equated to priests in Christianity and rabbis in Judaism. However, there is in fact no role like a priest in Islam. Each worshipper prays directly to Allah. Each member of the congregation, including the Imam, faces mecca and does the exact same prayer. The role of the imam during prayer is merely to keep the congregation in sync. That's it. Anyone who is Muslim and knows how to do salat can be the imam.
When praying in congregation, the women are always either behind the men (usually behind some kind of screen though not always) or in a separate prayer hall. The exact arrangement depends on the facilites available. Generally speaking Muslim women prefer a separate prayer hall where one exists. At the time of the Prophet there were no such facilities, the women prayed in the same prayer hall as the men (the hall was outdoors, more like a courtyard than what we think of as a prayer hall nowdays) - the men prayed in rows starting from the front, the women prayed in rows starting from the back. The reason why women are at the back is because when you do sujood (prostration) your bottom sticks up in the air, and it is considered that men are a lot more likely to be distracted by womens bottoms being in the air than vice versa. However there is a hadeeth about a time when some bedouin first converted to islam, and didn't have long enough garments to cover their awra (for the men, this is the area between the navel and knees inclusively) when they went into sujood. There was no-one that was able to lend them clothing so they had to pray like this out of necessity. (under normal circumstances you must cover your awra when you pray) The Prophet commanded the women to remain in sujood for the count of twenty each time, in order to give the men time to get up from sujood before any of the women could see their awra.
From this we can see the importance of not having distractions from attractive parts of the opposite sex whilst praying. The counter argument to this is usually that people should control their thoughts and their desires. Of course this is also true, but when you pray you should do your utmost to concentrate on Allah throughout the prayer, and anyone that has tried this realises actually its very difficult. You need to remove all distracting influences from the environment when you pray. Also, the rules on these matters need to take into account the weakest in the community, rather than being based on an ideal that not everyone will succeed in living up to. As men are more distracted by women than vice versa, men pray at the front.
As for the other roles often carried out by Imams, e.g. teaching, advising, giving fatwas (if they are qualified to do so) - all of these can be done by women if they are qualified. Scholars are the people who do these things (a mufti is a scholar who is qualified to give a fatwa) - and women have always been scholars. In fact the greatest ever female scholar was Aisha (ra) who was Muhammad's wife. More ahadeeth were narrated through her than any other of the companions of Muhammad. She taught, lectured and advised both men and women for many years after Muhammad died.