Ahmadinejad apparently got 62% of the vote, giving him the win outright. Frankly, I wouldn't be surprised if that was nearly true. But Mousavi is calling foul, alleging vote-rigging, intimidation, etc. The US has yet to make an announcement on the issue.
In response, Mousavi supporters have taken to the streets in Tehran. Buses are being burned, shops broken, etc. Tehran needs to avoid a total crackdown, but the Revolutionary Guard has promised to deal decisively with the problem. Additionally, cell phone services are currently suspended in the Tehran area, and websites related to Mousavi have been blocked.
The chance that this will turn into a full-fledged revolution is quite low, and without such a push, the protests are unlikely to go far. They may damage the regime internationally--or at least with the West--if their fury can provoke Western nations into condemning the outcome. But such condemnations are likely to drive Iran's policymakers deeper into partisanship.