The Sky News website is reporting that 30 people had to be rescued, and the London Fire Brigade Twitter account appears to confirm earlier reports that some passengers may have jumped into the river to escape. Thankfully, they follow up with a tweet suggesting that "there don't seem to be any serious injuries".
We'll report more details as we hear them.
UPDATE 13:30: The Sky News and Guardian websites have been updated with more details (and at least one rather alarming picture of passengers in the river and the boat filled with smoke and flames), indicating that the passengers were rescued by a passing tourist boat, before the fire was put out. Apparently two people have been taken to hospital suffering from suspected smoke inhalation.
UPDATE 14:00: The Huffington Post has several pictures of the incident as seen from the south bank of the river, and YouTube user Kaj Siebert has uploaded a video of the (now empty and extinguished) boat being towed away from the embankment by a fire rescue boat. (The first sound on the video is that of a small child asking "Why?". It's a good question, in the wider context of how such an incident could have occurred.)