In reply to yorkshire_lad2:
From their website:
In conjunction with Mike Berners-Lee we have produced a guide to keeping the day as low carbon as possible through travel, food and clothing. This guide will be sent to all participants on signing up to the event. In addition Another Way are planting 1700 trees in Matterdale in the Autumn in partnership with the Tree Council to offset the carbon footprint from the event.
...and...
It goes without saying that we would like all participants to follow the countryside code. Additionally we ask all Ambassadors and Supporters to follow main paths and to pick up any litter that they see during the day. Let's leave the fells in a better state than when we started.
I'm not involved in the event aside form reporting it here, but I'm not convinced its environmental footprint is a big worry. Sure, it'd be more 'sustainable' if all 600+ people stayed at home, but then it wouldn't be getting much of a message out...