Seems like a simple way to save the planet, make new friends, and connect with people in your community. 1 Sunday a year where people go to the church which is physically the nearest to their home, which for most people will mean it's within walking distance. If you live in a town centre, or the City of London, that will mean some fine calculations with a pencil and a ball of string, but shouldn't be too tough to work out.
The previous Sunday, get everyone in church who got there by car to work out how far they've travelled, and then you can calculate the carbon footprint saved by going local. An unintended side effect might be that certain communities find they have a viable resident churchgoing population, and they decide to get stuck in to where they live, rather than where they commute to.
The only problem I can see is that not every church has a service every Sunday. Our nearest church is in the village of Thorne Coffin (yes, really), and I think they only have a service once a month. It may also mean a very quiet Sunday for town centre churches, but the odd week off won't do anyone any harm.
Mentioned this on twitter and got a couple of retweets, so we'll see if it's got legs. Even if one community, or even one person, picks it up, that's better than nothing. What do you think?