Blog Action Day was a good example of a 'word of mouth' campaign that called thousands of people together through a collective interest (in this case, blogging).
The premise is relatively simple: if every blog on the same day has the same focus, more people (readers of blogs) will become aware, informed and interested in the issue.
It's a bit like running the same advert on every TV channel, at the same time (a concept you might better appreciate if you don't have cable or satellite TV!).
Does it work? Well, thousands of bloggers turned their blogs to the issue, or at least linked to the main website (Blog Action Day), or another relevant site of interest.
I would argue that the largest impact would be seen at websites where the focus is traditionally on other issues: by turning their attention to the environment, they create a new focus and have more chance of grabbing their readers' attention. Blogs that regularly report on environmental issues would be less likely to spur sudden interest in the underlying issues, though they might benefit from greater traffic as a result of all the publicity.
A global campaign like this is hard to organise, and the idea would quickly pale if used too often. However limited campaigns can be quite effective.
You could try a mini campaign - aim for a very selected audience (e.g. bloggers who already write about the environment) and ask them to write about a very specific issue on the same day (e.g. forestry). Make sure you have plenty of supporting material for them to draw on (statistics, images, quotes, etc.) and invite them to link to your site or another that is pertinent and authoritative.
Another idea is to adopt a rolling campaign, with no specific date, but which encourages bloggers to post a link, icon (with link), image or brief message when they can. Web circles and their ilk take a similar approach by networking like-minded websites.
For my own contribution, I am providing links to the UN's leading environmental organisations, for your further information.
UN Environment Programme www.unep.org
UN Framework Convention on Climate Change www.unfccc.int
Intergovernmental Panel on Climate Change www.ipcc.ch
UN Convention to Combat Desertification www.unccd.int