A wifi commons

As I sit here, my laptop can see 7 wireless networks other than my own - I suspect many of them have, like mine, been secured.

Here’s a question: what would happen if everyone with password-protected wifi in the UK reconfigured their routers so that up to 25% of their bandwidth was available unprotected to anyone who wanted to use it?

I don’t even know if that’s possible with current routers, but most of them surely have hardware that’s capable - and of making sure there was no route to machines on the the secure network from the public side.

Am I naive in thinking that this would create a sensible wifi commons across most major cities?

Update: James has pointed me to Fon who make a router that already does exactly this.