WWF's panda symbol is a registered trademark and is subject to copyright.
It must not be used or reproduced without express permission.
...but if the logo has already been incorporated by WWF in an image, banner or flash presentation, then that is OK - though you must never use such material to endorse or promote your site or products.
The letters “WWF” are also a registered trademark and must not be used to endorse or promote any products or services in any way, shape or form.
However, licensing and partnership possibilities are available... as you would have guessed... Please contact us for details.
Here's the legal fluff on the logo etc:
© 1986 Panda Symbol WWF - World Wide Fund For Nature (formerly World Wildlife Fund)
® "WWF" & "Living Planet" are WWF Registered Trademarks