Is it KFC or Kentucky Fried Chicken? Well in that case legally it doesn't matter because the same company owns both the name and the abbreviation. Of course for marketing reasons they have swapped from the full-name to the abbreviation and back but that's another story.
In the case of Windows Workflow Foundation we own the name Windows Workflow Foundation. However, we didn't trademark an abbreviation so there is no official abbreviation. Indeed we tried, albeit with a phenomenal lack of success, to launch the technology referring to its name and its name only. It was an interesting study in Microsoft and press culture to see people abbreviating to WWF with a few notable exceptions here and here. Given people’s natural desire to abbreviate the technology name to three letters and potential, untested, issues with the WWF acronym we feel compelled to strongly suggest you use the acronym “WF” for the technology only.
Remember its Kentucky Fried Chicken (Windows Workflow Foundation) not KFC (WF) so use WF when you are severely space constrained.