From Business Week comes this tantalizing tale of using dubious green credits (carbon offsets) that companies tout to make themselves look better to the enviros. See the story here. Then comes the observation that, uh, oh, Walmart leads the pack. Sorry Sam. Sorry Randy.

Amid the overheated claims, some corporations have made legitimate environmental gains. Wal-Mart Stores (WMT ) helped spark the market for energy-saving fluorescent bulbs by giving them top billing, even though incandescent bulbs are more profitable. Office Depot overhauled lighting and energy in more than 600 stores, contributing to the company’s real 10% decline in releases of heat-trapping gases. Dow Chemical (DOW ) and DuPont (DD ) have significantly trimmed their actual emission levels. But there is still reason to worry about long-term commitment. Dow says it invested $1 billion to help achieve reductions of 19% between 1994 and 2005.

So called REC’s (renewable energy credits) have found a critic in the Aspen Skiing Environmental Director who says that “REC’s have as much effect on the development of new renewable-energy projects as would trading rocks, IOUs or pinecones.”