If your personal preference is to use environmentally friendly ETFs whenever possible, there are a number of them which might be of interest to you. Of course, just because an ETF is exposed to that area, does not mean it is an appropriate investment at this time.
ETF Trends had a listing of 9 ETFs that are involved in that sector, of which 7 are listed in my weekly StatSheet. If the clean-tech sector is an area you are looking to invest in, you may want to check out this list, which includes my Momentum (M-Index) numbers as of 10/8/07:
While we currently have no holdings in any of the above ETfs, this may change if they move up higher in the sector rankings.