Larger image:

Not Clarice Black and probably a custom design.
The base letters may have been derived from a font like Radiant Black
http://www.myfonts.com/search/radiant/fonts/
or Castle Ultra
http://www.identifont.com/show?2MG
using overlapping Vs to make a W with the center strokes meeting in a "T" to the left, and rounding the corners of the Ws and F to make soft and "friendly" letters.
Don

Hi damn1990,
Definitely not Clarice Black and Don's probably right about it being a custom logotype (registered ®), but pretty simple though - all straight lines and circles.
There is some info and a video about the WWF logo and it's development here:
http://www.logodesignlove.com/world-wildlife-fund

I also noticed your other request regarding other "WWF lungs typo" info, so I feel I should refer you to the info available from their website in respect of using site content. This is the legal bit, I guess!
http://wwf.panda.org/using_site_content/

Hope this helps
Doghouse

Can't say exactly why but looks like somebody played with Peignot and the more I look at it the more I think it's a manually rounded version of it (which to me looks very bad)...