I've been mapping more of the area between Carshalton and Sutton this week, and I'm now nearing completion of this block. I've left the area of highest population density to last, as it will take the longest.

Talking in IRC yesterday, it was revealed that there's been a changes method in the API for around 2 weeks now, un-announced. Someone from the tiles@home project quickly managed to use it to seed the tile request list every few hours.
It's really great to finish mapping at 1am, go to bed, wake up and see your changes in the morning, without having to manually request them :)