Solar Web Host, based out of the UK co-locates their servers with a company in California (could it be AISO.net, also located in California, US?) Their website is a bit harder to nail down in terms of specific claims about environmental friendliness but here is what they say:
In their words:
SOLAR POWERED WEB HOSTING SYSTEMS
The datacentre we utilise for this option is in California east of Los Angeles. The data centre is powered by 120 solar panels which generate all the electricity. The Solar is owned and operated by the Datacentre to power California’s first and only solar-powered hosting company. (please be aware that there is a 10 hour time difference to the US so set-up times are longer and accounts can take up to 72 hours to activate).